I played it last night (PS3), and I'm enjoying it. Gameplay wise it's very similar to the first one- the biggest changes are L3 sprint and L1 zoom aim- almost functions as a lock on. You'll feel right at home.
People are right that a lot of the textures are pretty low-res, and I for one really miss euphoria. BUT I've barely scratched the surface of the game- no grappling hook, no helicopter, no real firepower. The big graphics test, for me, will be flying a helicopter over the jungle then decimating a city with a hail of missiles. Who cares about a splotchy wall texture when the point of the game is HUGE FUCKING EXPLOSIONS?
I think it's really fun- there's a lot of depth to the PMC aspect and the resources you need to collect. As with the first one, you can tell the designers are military nerds with the amount of details in the vehicles and weapons.
Other stuff- there's quite a lot of traffic on the street at all times, and there are a lot of firefights happening kind of randomly around. This was one of my favorite aspects of the original, and I'm glad to say they've ramped it up big time, it seems.
I think as I get into it (the game tells me I'm at 0%) I'll like it more and more. Mercs1 fans, have no fear!