I must admit I'm very picky when it comes to F2P MMOs and so far I'm enjoying SoF (Lv23 Spearman). I played the latest popular titles and got bored in no time (Trickster, Voyage Century, 9Dragons, 2Moons...)
I agree with cyber7chink, it's nice to have little goals to help you grind, and the fact you only get 1 (meaningful) quest every 2~3 levels (giving really nice rewards). God I hated the quests-fest in Cabal Online, too many quests, running around town > quest spot was worst than plain grind.
The graphics are decent if you put everything on max (including glow), better than most F2P MMOs if you are into Chibi/Super Deformed design (<3).