I just switched from Speedplay X2s to Shimano Ultegra SPD-SL (PD-6620).

While I loved the float of the Speedplays, and they were absolutely effortless to clip out of and into (friends with one-sided pedals sometimes fumble at traffic lights to clip back in; not me in the X2s), I found myself developing 'hot spots' on longer rides (45+ miles or so). Folks claim that the Speedplay pedal has a large platform because of the way the cleat clips in with the pedal, but I could definitely tell I was on lollipops and my feet really weren't supported.

After getting really, really painful hotspots during a Half IM, I decided the Speedplays had to go. Folks at the LPS recommended the Look Keo CroMo (which you can find on sale online), but I wanted the widest platform so I got the Shimano SPD-SLs. So far I really like the supportive and wide platform. Not nearly as much float as the Speedplays, but still a bit with the stock cleats. I'm still getting the hang of clipping in since the pedals are one-sided. Sometimes I have to look down to ensure I'm clipping in right. And despite having the tension set to the lowest setting (I think), it takes some effort to clip out.

There is a 105 version of the newest SPD-SLs for 2008, the PD-5610. Good bargain, IMO. Weighs slightly more than the Ultegras but cost considerably less.