It's amazing. I just completed it a couple of days ago. Overall, much better than Chains of Olympus.
Graphics are ridiculously good for a handheld. I was absolutely amazed by what they managed to acheive on the PSP.
Gameplay is far more fluid with some brilliant weaponry and magic attacks that are actually very useful (agree with OP on Theras Bane).
More stuff to do (challenges etc.) after completing the game than Chains of Olympus.
The storyline was covered very nicely. It's a bridge between GoW1 & 2 and I actually feel it makes it easier to relate to Kratos.
Soundtrack is very good and fits well with the game. I downloaded it on PSN early so now have the soundtrack on my PS3 as well as the GoW theme.
The only slight downside is I think the puzzles can sometimes be a bit too easy, but this also prevents frustration, so perhaps it's also a plus.
Overall, one of the best games out on PSP or any handheld for that matter. I actually prefer it to GoW1.