Like the other reviewers on here, I'd downloaded the demo and was very impressed with it - sure it was never going to be as good as anything on the PS3 but it comes pretty damn close imo.
I actually paid a few quid more to download it from the PSN network as I was too impatient to wait for it to arrive properly and so I'm not certain if the minor niggles I've found with the game are the same on the UMD version.
The main gripe is the cut-scenes - they are fairly jerky when played from the memory stick - I dont know if this is because of the hardware limitations of the memory stick or just the game in general, but that's pretty much the only complaint I've got with the game.
Graphically it's brilliant for a handheld (I think so anyway) - enemies are detailed and there are some very well done large boss-type enemies. There's the ability to command a mech at certain points of the game and this serves as a nice break from the standard running and gunning.
The game is pretty hard in places - especially in areas with little cover - you'll find you'll still get hit even when behind a small wall - and you will die a lot in this game - but perseverance pays off in the end.
Sound is great too with suitably dramatic scores which changes with the action - no complaints in this department either.
All in all - I'm happy with the game - I've been playing if fairly regularly for about a week now (yes - it can be THAT hard!) and I'm fairly sure I'm close to finishing it - but even so, it's still got great value for money with it's completion time. For £17.99 this game is an unbelievable bargain - all I can say is buy it now or you'll regret it!