Speedplays are getting a bad rap here from what sounds like folks who have never tried them. And some of the comparisons seem a bit unfair. I am no Speedplay evangelist (as I said above, what works for me may not work for others), but there seems to be a need for a bit of "correction" here:
I cannot remember the exact weights, but when I switched from SPDs to Speedplays, I weighed both the cleats and the pedals (out of curiosity, not because I cared that much). My X-3 (the heaviest Speedplays) weighed just above 1/2 my SPDs (an older, cheaper model). The SPD cleats were indeed a few grams lighter than the the Speedplay cleats. I doubt there is much difference between the Speedplay cleats and Look cleats.
The end result was that I saved at least 40% on weight. I noticed the difference immediately. I cannot confirm, but when I was shopping for new pedals, I noticed that Speedplays were listed as weight per *pair*, while all others were listed as weigh per *pedal*.
Furthermore, IslandRider implies a comparison of a Titanium SPD with some unnamed Speedplay (but, based on the weights given, presumably the Cr-Mo, their heaviest). If you want to go with Ti, Speedplay makes a Ti pedal that comes in at about 150 grams per pair (and the cleats can't be more than 50 grams per pair).
As to the dirt issue: I can only speak for the "X-" road pedals, and, yes, a lot of dirt would prevent proper engagement. But, keeping them clean is a non-issue. I look at them after every couple of rides. I find it necessary to brush them out only once every couple of weeks.
I have never walked comforably or confidently in any cleated cycling shoe. Frankly, my SPDs left scratches in the marble floor in my entry way, requiring me to put them on outside (hardwood floors anyone?). The Speedplays do not. And, after all, does one buy cleats and cycling shoes for walking?
As I said before, they cured my knee problems and the icy float feeling went away fast. I have not tried Look or Lake pedals, but as far as I am concerned, SPDs are a lousy system--hard to position correctly, they create a minimal platform (which, admittedly, may be a non-issue, but I didn't like it), and the float is compromised by pedal's spring tension.
I would suggest, don't judge a pedal before you've ridden a few hundred kilometers on it.
Cheers,
Jamie