While it might not be as polished as Halo 3 (or CoD4, which is coming to the PS3 anyways), it's a heck of a lot of fun. It's cheap, bundled with a good headset. No reason not to get this game if you're getting a PS3.
It shouldn't be the primary reason you're getting a PS3, as Halo 3 might be the better online shooter (though I personally don't care for Halo). There are a lot of other reasons to want a PS3 though, so I can only recommend it. Heavenly Sword (rental at least), Ratchet, Uncharted, Guitar Hero 3 & Rock Band (both which are multiplatform), CoD4 (multiplatform as well), HL2 Orange Box (multiplatform), Killzone 2, Metal Gear Solid 4, Final Fantasy XIII, White Knight Story.
Do it!