Arrgh! Mark! said:
I think it requires a GM who is a) fast as lightning and b) knows the rules inside out, and the players abilities inside out. Also, c) a good, interesting plot would be helpful - we unfortunately didn't have any of those benefits.
Interesting you'd say that! Terramotus is my IH DM, actually, and I agree, if he didn't know the rules so well, it'd be pretty miserable. He was too nice to say that I was the DM for the ill-fated pirates campaign he mentioned or that it was basically my lack of experience that doomed it. I'm nowhere near as rules-brained as my players are and it was painfully obvious from the beginning that I was in over my head.
I'd been running a very fast-paced, very RP-heavy monster hunting D20 Modern campaign for about a year and a half at that point, and I was completely unprepared to run a game that required me to know more than a few basic DCs and how to read a statblock. Preparing and running games under IH was a LOT more work than I was ready for.
It was especially frustrating because I could picture the scenes in my head (say, the grand swashbuckling fight in the enemy's fortress dining hall while flames from a kicked-over candelabra race up the drapes) but I wasn't skilled enough to actually execute them. I didn't even really understand some of the key concepts, like zones, until we'd all pretty much decided it just wasn't working.
A lot of the 'cool' features of IH -- stunts, zones, token pools -- really only shine if the DM puts a lot of prep work into it or the players are really, really inventive and the DM is willing and capable of being flexible and creative (and in some cases, has seen the same movies you have).
I don't think the system is overly complicated, though, and I'm a four-pages-of-backstory-what's-BAB-mean-again? type player usually. I haven't had any trouble running my executioner character at all, and, in fact, he's probably my favorite character ever so far.
One thing I did notice, though, is that seemed as if the IH system really lends itself to either an intrigue-heavy campaign OR a combat-heavy campaign, and you have to be careful that all of your players are on the same page as far as what their characters will be doing.
My executioner was originally a thief, for example. I loved having all the skill points, but my combat abilities were so poor I was basically standing around any time we weren't talking to someone. And some of the feat trees tailored for the thief are ill-suited to a player character (at least one feat tree is designed to make your character the power behind the throne; not particularly useful when you're lost on a mountain and a titan is trying to kick your teeth in, IMO. Maybe in preventing the situation, but then you're a Grand Vizier, not an adventuring hero).
One of the other players helped me swap my backend from thief to executioner, and, while the character didn't change substantially story-wise, he really became a lot more fun to play (I had no idea bouncing all those sneak attack dice would be so satisfying!) in the context of a gritty, Special Forces-style, low-magic campaign.
So I don't think IH is a good system necessarily for learning to game, or one for every plot or setting, but in the right hands and with the right setting it's been a lot of fun. And the combat speed really picked up after the first few sessions, once we got the rhythm of combat down and felt out how our token pools could be used most effectively.
And if I had to sum up our Rome campaign, I would have to say, best campaign ever.

I'm madly happy with my character, who I manage to get at least a little RP in for every single game, and the other players, who tend to be a bit more combat-minded, seem pretty happy with the three or four fights we somehow get into every session.
(Sorry, looks like I wrote a novel!)