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'll get off my soap box now.