2-FDCK is much like ketamine, though not as good, in my opinion. Messes with your motor function a bit more, and harder to get into that euphoric head state of regular K. The whole vibe feels "dirtier" to me, both in actual taste as in the experience itself (more intoxication / less euphoria). I'm talking quite subtle differences here, and I take ketamine as a party drug. Can't say anything about the truly dissociative / holing properties of 2-FDCK.
As for 6-APB -- I stopped taking MDMA at raves because it stopped doing anything for me, except for making me all hazy, intoxicated, and inwardly oriented. Now 6-APB really feels like a good replacement, it's similar, but the effect is more stable, instead of coming in peaks, and longer-lasting. If dosed and timed correctly, you need only a single dose, and it's this habitual MDMA redosing that causes the infamous Tuesday blues and all that shit. Not with 6-APB. A notable difference is that 6-APB sort of alternates between energetic and laid-back, the latter feels kinda like 2C-B, can induce yawning as well. Subject to dosage, of course, I'd say MDMA has a bit more kick while 6-APB is more gentle.