Now don't get me wrong, Battlefield 2 is a good game, it's combined arms gameplay is very enjoyable and the infantry combat is mostly fine,but it isn't without its problems.
Big maps like Dragon Valley make people left without vehicles feel useless. Vehicles have some balance problems, while tanks and other land vehicles are mostly fine most air vehicles are hopelessly overpowered and a good helicopter pilot and gunner can sometimes be an unstoppable deathmachine. Infantry combat also has an unfortunate habit of turning into nade spamfest on some infantry maps.
While Battlefield 2 isn't the best game of the series,(2142 takes that spot imo) i still recommend to buy it if you haven't.(its only like 5 € and has awesome mods too.)