I've enjoyed each expansion so far.
B2K was awesome because BF2 maps are awesome, it's probably the most fun expansion in terms of gameplay, imo.
Close Quarters was O.K. I do enjoy a good game of Conquest Domination, it's very fast paced and high action. The CQ guns are pretty cool, especially the SPAS-12

It does break with traditional battlefield but I don't really see what's wrong with that, diversity is good.
Armored Kill is exactly as advertised, it's all out vehicle warfare. Most maps have between 6-8 tanks for each team, mobile artillery, attack helicopters, scout helicopters, jets, AC-130. It's pretty awesome but that much armor on a map can very easily lead to imbalance. Once a team takes the majority of the caps and thus controls the majority of the armor it can be difficult to come back. Armored Kill also has a proper air ceiling, none of the sub 1000 BS that you find on other maps, flying through a cloud layer is pretty cool.
So in my opinion each of the expansions has been worthwhile, they each bring a different kind of gameplay to the the table and give you options.