There are plenty of threads about this already, I'd recommend checking them out..
Regardless, I would recommend at the very least you rent the game. If you're a fan of the older Call of Duty games (CoD 4, MW2, etc), as well as Battlefield, I think it's safe to say you'll enjoy Modern Warfare. The game hits a pretty specific niche market in my opinion, but I think it is a ton of fun to play. The depth of weapon customization is unlike anything seen in any Call of Duty game before, and Ground War is a great gamemode to sink a lot of time into.