Not sure why people are complaining about the voice-acting -- it's excellent (remember, the setting calls for 1951 British English, and the dialogue is true to that time-period). The characters are deeper than the average shooter, the cover and fire system work as advertised, lots of crazy weapons to play with, the atmosphere and cut-scenes are first rate. All in all, Res:Retri is a gem of a game, so if you're into handheld shooters or enjoyed any part of the Resistance universe, buy it.
One quibble: yes, there's no second analogue stick on the PSP, making aiming P...A...I...N...F...U...L. Not broken, you just have to re-learn aiming. However, Insomniac wisely included the "PSP Plus" option -- if you have a PS3 and a copy of Resistance 2 (another game I recommend whole-heartedly), start up R2, go to the Options menu, plug in your PSP via a USB cable to your PS3, and click on the "PSP Plus" mode. Hey presto -- you can now play Retribution using your standard PS3 controller. Watching the small screen can be difficult, but you can plug in your PSP into your TV and play it on a widescreen. Also, there's "Infected" mode, giving you health regeneration (which I prefer).
Last but not least: lots of intel and storyline, fleshing out the Resistance world, which I enjoyed immensely.