I love Resistance 2. I (slightly) prefer the controls to the controls on Killzone 2. That said the Killzone 2 controls are not bad and take very little adjustment.
I've played through the campaign on easy, normal and hard... lol I think it was hardest on Easy, it seemed like there were more enemies and they worked against you more? I don't know..
Anyway, where Resistance 2 shines is the multiplayer which can have up to 60 people playing at once in a match.
If you're playing competitively, you can take part in skirmishes, deathmatches, team deathmatches, Core control (capture the flag), I'm thinking there is another mode but my brain isn't working right now
Playing Cooperatively, you can choose from one of three classes.
The Soldier who is intended to be your chimera killer and has a shield that others can duck behind.
The Medic who keeps your team healed (regen doesn't work on coop)
The Special Ops who pulls double duty as sniper and ammo replenisher. In lower levels he is only good for ammo.
I have been playing coop mostly (as of late) and sometimes you get stuck with people who don't understand their roles assuming (I suppose) that everyone needs to go kill crazy. Thats a failed mission waiting to happen. Everyone needs to pull their weight with the soldier doing the killing and everyone else backing each other up.
While I love Resistance 2 and it's multiplayer modes, when I first played Killzone 2 I was in love with it's multiplayer mode. I love playing the skirmishes on Resistance 2 and when I was doing the skirmish on KZ2 it was all kinds of fun.
I just wish my team would go pick it up already.