My 2nd and 3rd most played maps are SF maps, 5th is AF and 6th is an EF map
My 2nd most played map is "Iron Gator" on SF. The 'mission' is clear-cut : MEC attack the carrier and USMC defend it until the bitter end. First half of the round usually consists of the MEC relentlessly trying to get a foothold on the carrier whatever way they can, attacking from air and sea. After the first flag is capped, cue the sweeping firefight along the ship down the hallways and deck. Brilliant blend of sea, air and close-quarters battle - all in the same place.
"Warlord" is my 3rd most played - it's basically the SF equivalent of Karkand, but faster (quad bikes, triple-gun vehicles, 4x4's, zip-lines and grappling hooks to get you around the map faster) and with brilliant chaotic firefights in the palace at the north when you get opposing forces holding flags on each side of the building.
AF is great for part-time jet and heli whores like myself. Take "Operation Road Rage" - two A-10's and their amazing gattling guns, two SU-39's, no AA missiles so you _have_ to use the guns in dogfights (cue very long dog fights, flying through bridges and hangers trying to shake that A-10 off) and the super-manoverable "little bird" choppers with chain guns and seats on each side. I've got over 80 hours on that map and counting...
Operation Smoke Screen from EF is my 6th most played - I think they got the maps for EF and AF mixed up, because I've never seen tank battles like the ones in Smoke Screen on any other map. If your team is doing well you can end up with 8 tanks, 4 jets and 4 attack heli's knocking on your enemy's base - It's an intense map.
In my opinion SF and AF are well worth the dosh - I think they have some of the best maps in the game, I just wish more servers played them (especially the case with AF and EF)
Taking a price of 25 quid for SF, and a tenner for AF - it works out at 6 pence per hour for SF and 9 pence per hour for AF for the time I've played on them - a good deal in my book!