Definitely give it a go but don't expect it to play like a FPS just because it uses the same controls - it's a military simulator, and therefore is a lot less forgiving!
A stand-alone expansion is coming out next month (June 29th) called Operation Arrowhead, set in the Middle-East. I'd still get the original ARMA 2 - I can't get enough of roaming through 225 square km of beautiful countryside.
And yes, it runs like a dog but the developers release beta builds regularly between patches which address issues. The latest patch is certainly A LOT smoother than what comes on the DVD, and lots more performance enhancing is rumoured for Operation Arrowhead.