I built an Invisible Blade recently and I found myself thoroughly underwhelmed by pretty much every aspect of the class. It's currently about as close as one gets to the "Arcane Trickster" aesthetic, so I was quite determined to make it work, but in the end it really just felt like a completely pointless addition to my build. For context, I was previously a DEX-based rogue-heavy build with dual wield feats and a downright respectable AB. With 6 APR before Haste, I figured I could utilize the Bleed mechanics to their fullest potential. Before I started playing this character, I couldn't find a good reason to take 4 or 5 levels in the class instead of 3, and then as I was playing with the class and realizing just how negligible the damage it offered truly was, I found myself struggling to justify taking any levels at all. In the end, I dropped the levels from my build entirely and replaced them with something, anything, that gave me a bonus feat.
Class Features
Invisible Blade's mechanics are based around stacking Bleed counters on your enemies, and then using the Eviscerate ability to expend those stacks to deal extra damage. It also has a breach ability, on a 3-minute cooldown, that additionally subjects the target to 20% spell failure (okay, apparently double-checking the wiki it's JUST -5 Lore/UMD, it's even worse than I assumed). There's a third ability, Crippling Throw, which is a ranged touch attack that slows movement speed and adds a negligible amount of bleed stacks - but on top of having an Intelligence-based DC your enemies need to fail, it has truly abysmal range and seems to largely be a waste of time to use.
In fact, all these abilities are sort of a waste of time to use. None of them are instant actions, so your character awkwardly stands around after you trigger it and then spends the next turn executing in slow motion, and then they each go on a considerable cooldown so you just get into the habit of forgetting they exist. The only ability I utilized semi-regularly was Arcane Bleed, saving them for dungeon bosses with damage shields. Functionally, however, it's worse than just using a breach wand because you have to be in melee range and a lot can go wrong when you're in someone's face. If it came to PvP, I wouldn't even slot it in my hot bar.
In order to use these already underwhelming abilities, you have to use weapons that are one size category smaller than your character - thus, your average human can use small weapons, and your average halfling is stuck with tiny weapons. This limits the diversity of one's gear options, if you want to actually perform the features of the class you spent 3-5 levels investing in.
And 5 is the maximum amount of levels you can take in the class, within which all your key abilities are earned by level 3. None of your class abilities are tied to IB levels, except that when you take the full 5 levels, you get a little extra Bleed damage and a little extra Eviscerate damage. Level 4 is a dead level in every sense.
Gameplay
I took IB levels early on, thinking that as a Rogue it might be a nice damage supplement for if I'm ever running solo or if I'm fighting a sneak immune enemy. Turns out, there's considerable overlap between sneak immunity and bleed immunity! And it also turns out that I'd rather experience a kidney stone than solo grind, so that was also never a relevant consideration.
As was mentioned, the IB class features were rarely utilized in my journey. If I triggered Eviscerate on an enemy I was already sneak-attacking, I wouldn't even notice the damage. Breaking stealth for a low-range crippling throw that would miss 7/10 times despite my reliable AB on a touch attack is simply an act of self-harm. I thought Arcane Bleed might be sort of nice for the 20% spell failure chance, but it turns out that aspect of the ability was a hallucination I conjured as a coping mechanism for how pointless the ability is when breach wands are so easily accessible.
The damage is negligible. The abilities are forgettable. I found Dirty Fighting was far more reliable and far less unwieldy, so even though the DC wasn't great for me, I found myself using that ability far more often than any IB skills, simply because it was fun. IB really doesn't add anything fun to the kit. Eviscerate is a trap, because it seems to me that you do much more damage simply letting the Bleed ticks drain your enemy while you take the round you'd otherwise waste on the ability to simply attack.
Through another lens, it also just doesn't add anything new or unique in terms of class mechanics. Functionally speaking, it's no different from playing a rogue. It even synergizes with Rogue in many of its features - features that require Rogue levels to begin with - which, if you look on the Wiki, take up more space on IB's page than the descriptions for IB features do. Since dropping the class out of my build, my play experience is utterly unchanged. If anything, it's improved, because I got one extra feat out of the trade, and when you consider how feat-starved a majority of DEX builds are, this is simply always going to be the superior option.
Roleplay Identity
IB doesn't add much in terms of mechanical identity or variety in play. What's much more egregious, in my eyes, is that it furthermore doesn't add much in terms of roleplay identity. Even with a full five levels in the class, your character will never be recognized as "Joey the Invisible Blade," because there's nothing truly identifiable about IB. You have three combat abilities, a DoT on hit, and that's all there is to it. Compared to the strong identity provided by rogues as the grenadier, the infiltrator, the conniving mercenary - or compare it to taking just 5 levels in another prestige class, like the Shadowdancer with their shadow, or the Blackguard with their backstabbing smites and mastery of fiends, or the Loremaster with their ability to make the game a little less fun for everyone around them. Invisible Blade is, by design, a supplemental class. It doesn't stand on its own, it doesn't contribute anything unique, it's just a new, weird damage delivery vehicle that strikes me almost like a test run for various mechanics that might be implemented elsewhere. It's utterly uninteresting to play, it's sort of impossible to roleplay due to its lack of conceptual substance, and I personally consider it to be a trap.
Missing Pieces
As I mentioned, I'm playing a dual-wielding DEXer. I need feats like I need air to breathe. The fact that IB offers none was essentially the killing blow that eliminated all my interest in sticking with the class. Give level 4 a bonus feat so it's not such a useless level - heck, give it the Rogue bonus feats to make it even more attractive! - and you might even see a little more investment into the class.
Furthermore, the abilities really need to be as instant and easy to use as Dirty Fighting - and the cooldowns could really use some renewed consideration. The breach is simply not useful enough to be locked behind 3 minutes, and the time it takes to activate any of these abilities truly feels like I'm losing more damage than I'm gaining.
In PvE, I kill most of my enemies in 1-3 rounds. My build is so far untested in PvP, but I find most instances of PvP lasts 5 rounds or less. The DoT damage offered by Invisible Blade might be considerable if you stack it up and let it whittle an enemy down, but considering the pace of combat, it literally doesn't have any time to shine at all. I don't do spreadsheet math so maybe I'm just not giving the class's damage its due credit, but I stuck with it for 30 levels and I pretty much never noticed Bleed damage making any difference at all. I kill my enemies at the same pace now as I did then. It needs either more utility for its abilities to make some sort of tangible difference in a fight, or it needs more front-loaded damage. I don't necessarily know that it needs both.
AB or damage bonus or increased crit range on IB-compatible weapons. Right now, you can only be punished for NOT using IB-compatible weapons, rather than inventivised to use the weapons for your class. Graciously being granted access to the abilities you already know as part of taking the class doesn't qualify as a reward, it qualifies as an annoyance. I don't "specialize" in these weapons, I simply forget everything I know about arterial anatomy if I put anything else in my hand.
It needs something identifiably and uniquely IB. Given the name, something involving concealment (maybe +5-10% concealment per class level or something insane like that) or invisibility (greater invisibility on CD) or just something, anything weird and cool with a nifty VFX to demonstrate to an audience that you're a very unique and special snowflake.
In Conclusion
IB is boring and I don't enjoy playing it and to be honest I'm not even exactly sure who this class is meant for. I want an Arcane Trickster, but that doesn't exist (although I have a design doc in progress for how I'd make AT a reality, coming soon to a suggestion box near you). This class isn't even the poor man's Arcane Trickster, it's more like the rich man's Rogue - because only those who are disgustingly wealthy in feats and spreadsheet know-how can reasonably afford to invest in this class. I now know that ain't me.