I use speedplay frogs, you use them on mountain bike shoes, so you will be able to walk on the shoes. On mountain bike shoes, the cleat is recessed up into the shoes - so sometimes you hear a clicking sound if the cleat hits the pavement, but you can still walk on them.

I started off with them because I knew I had knee cap tracking issues, and they have a fair amount of float... they also detach pretty easily, you just kinda turn your heel out. You just sort of turn your heel out slightly and they pop off.

A lot of people seem to like the crank brothers systems as well.

I can't say that I've really tried anything else other than spd for a few days - then I went to frogs because they were easier on my knees and haven't had a reason to try otherwise.

Actually, I've got a set of crank brothers pedals to put on my mountain bike, because I think the frogs detach a little bit too easily for serious mountain biking... but then I'm not doing the latter, so I haven't gotten there yet.