i never played first game i will buy first one
I rather enjoyed the 1st one. 4 characters with different play styles, MP with bots (back on the PS2, I rather love this and still do when shooters do it as I don't have to put up with the usual MP crap of campers, people using the same weapons/classes, etc.). I know the game took a lot of flak though due to the media hyping it up as the PS2's 'Halo-Killer'
EDIT: Also loved the dual purpose/secondary fire setup of the weapons.
Kind of wish they'd expand the IP outside of just the FPS area though. I think the series has a great atmosphere and story and would like to see it branch out a bit more to further delve into that.