Speedplays are good in that there is no resistance when clipping in or out, which can hurt some people's knees. I have tried them and don't like them; you can easily slip right out of them (although that is more common when using them off road as you throw the bike around, jump, etc.
If you are having knee problems (such as tendonitis or Chondromalacia patellae) I would highly suggest refraining from riding fixed. Your knees are valuable, if you do not let your current issue heal, you could really be setting yourself up for long-term problems down the road. It may not be as cool to run a freewheel, or even as fun, but if it let's your knees heal you should do it.
I run Frogs on my fixie, have had them for close to two years now. I absolutely love them - mostly because I ride in Boston traffic and I like that clip out is effortless and can be instantenous. I find them much easier than SPD's, even if the tension is really low. I originally got them because I wanted the float too.
(Oh, and if you ever happen to wipe out around a corner, they slide across pavement really well.)