Though the sadistic difficulty will likely turn off casual fans of the genre, Liberation is a great shooter for your PSP
For one, the aiming mechanic would've benefited greatly from a second analog stick, although the biggest offender here is the sadistic difficulty and flawed save system. While Liberation is fairly linear, the constant waves of enemies will make it quite challenging to progress through the game, and some of the boss battles (the last one, in particular) will test the patience of most players. As satisfying as it is to beat a game with this level of challenge, I found myself getting frustrated far too often to call it a 'fair' difficulty. The levels are also fairly long (some of them took me roughly an hour), but since there is no mid-stage save, you must wait until you beat the level to save. The checkpoint system is nice, but a quick save at each checkpoint would've made this feel like a much more portable game. Killzone: Liberation is not a game for quick bursts of fun by any stretch.
Still, if you're looking for a challenging and well-made tactical shooter for a long bus ride, Liberation is worth a purchase. In spite of all the frustration, I had a fun time with Killzone: Liberation, and I'm glad I gave it a shot. You can find this game for just a few dollars these days, so I'd definitely recommend getting a copy if you're up for the challenge.