I've been using Speedplay X2s since 2002, after about a year with spd pedals. My knees love the float. Never have any trouble clipping out. I do have a little trouble clipping in my left foot. I had a shim under the cleat and it was screwing up the spring, so I took the shim out and it's much better. I was considering getting the light actions, but I have heard that you can unclip unintentionally and that does not thrill me.
My other bikes have campus pedals with spd clips on one side and flat on the other. It feels very "constricting" to ride without the float of the Speedplays when I ride my hybrid or mtb.
Get the cleat covers. I don't know how anyone could even attempt to walk without them. Plus, even the tiniest bit of dirt will mess up the cleats and you won't be able to clip in.