Killzone: Liberation for PSP Reviews by Gamers

You do not need to play Killzone on the PS2 to understand this game. The underlying story is one we've all heard before (think "Medal of Honor" in he future, except the Helghast are the new Nazis). In fact, crossing over into the isometric stance from the run-of-the-mill first person shooter, this game really distinguishes itself apart.
The production values are top-notch, from the opening cinematic and the sound.
The different weapons allow different tactics, from the general-purpose assault rifles to long-range (but slow) sniper rifles to short-range (but fast) machine pistols. You'll need to decide too because the enemy A.I. will not wait. They are aggressive, fight in groups and take cover (something certain first person shooters haven't mastered yet).
You can man machines of war as well. The tank is particularly satisfying, as you have both machine guns and cannon. Gun turrets are fun too.
Some light RPG elements are incorporated via mini-game challenges that earn you points towards new or enhanced abilities (e.g. carry more grenades which is very important).
Speaking of grenades, they are very easy to use and very satisfying to see several Helghast flip through the air ragdoll physics style. You'll hear "Grenade!" before it's too late.
Multi-player deathmatching is a new experience, especially from the non first-person perspective.
My only qualms are that your character can't jump (although he dodge and strafe). Also, a default pistol sidearm (even limited in ammo) would be welcome. The firefights get hectic and sometimes the assault rifle ammo looks a little low, worse if you have the low-capacity sniper rifle (even though there are weapons literally everywhere). The A.I. at times seemed a little too smart, as they sometimes saw my unseen outflanking maneuvers.
Overall, this is one of the best games available on the PSP.