I still play Special Forces, about 1/3 or more of the time I play BF2. There is only about 40 SF servers that show up after my filters (ranked, no password, not full, not empty, 32+ player maps, etc) so that is kind of disappointing compared to the thousands of regular BF2 servers, but they are the same servers that show up that I play at often. So I rarely have a problem finding a game. It kinda sucks that servers mostly only play Warlord and Ghost Town, since I like all of the SF maps pretty much, and the night ones are cool just RARELY get played.
I'd imagine the booster packs are the same way, but I mostly like SF for the maps (and tier 2 unlocks of course!) so you don't get to play them as much as regular BF2 maps.
You'll need BF2 for SF, so you could start with BF2 for $25-30, or I've seen places like Walmart bundle BF2 + SF for $40... may as well get this if you are going to get into BF2.