I've had this game for a few weeks now and thought I'd give it a good play before jumping to any conclusions over what I think of it.
The flaws which reviews have pointed to are there. For example, the control system is different to any game I've ever played, and it is fiddly. You do get used to it, although never to the extent where you really feel like you've mastered it.
Also, it does feel a tad easy. There are times where you get killed and begin to think that it may finally get a bit more challenging, but mistakes are easily avoided on second attempt. However, there are indeed 3 difficulty levels (easy, normal and hard), and I've only been using normal. I am not yet qualified to say whether or not the hard mode will challenge.
The other big let down for me is the absence of a multi-player or online mode. Although I am more of a single player game fan, this just feels like a bit of a kop out for a title which should have been a blockbuster release.
They are however the only real downside. The truth is it is a thoroughly enjoyable game. I have been a huge fan of Iron Man comics for years, and thought the film was awesome, and having hoped for years for a decent Iron Man game, this hasn't really dissapointed me. Being inside the suit and flying around shooting stuff is great fun, and the graphics are superb. A great showcase for the PSP's power.
3 Stars I think is an honest representation. It's fun, and well worth getting if your a fan, but it looses one star for the flaws I mentioned, and another because I can't help feeling it's something of a missed opportunity. A character and concept as great as Iron Man should have inspired a classic game, not just an enjoyable one for fans. For example, take the timeless Goldeneye. A classic, genre defining game that you didn't need to be a Bond fan to enjoy. It would be unfair to expect anything as monumental as that, but they have missed an opportunity for a game with wide and long-lasting appeal, rather than one that will earn a nice chunk of revenue to add onto the movie's success and merely satisfy fans.
Overall, if you're in two minds over buying it, I'd say go for it, you'll enjoy it. Just don't expect to be revisiting it in a few years time.