In the 1990s I switched to Speedplays for a while. The knee problems went away, and I realised pedal float was a good thing. But the Speedplays always felt a bit unstable to me and to be honest, my results were not as good with them. I felt like my foot was wobbling all over the place - up, down, side to side. To be fair, Speedplay has revamped their design quite a bit and their success under people like Cancellara is undeniable. I haven't tried the new versions.
This is what Keo Carbons look like
after a couple of years.
Note the wear on the pedal over the axle.
The cleats do wear out quickly if you walk on them, so the little cleat positioning tab that Looks offer (position memory) is a great feature, provided your shoes support it with the extra hole in the sole. Basically, you position your cleat, then tighten down the little tab. When you need a new cleat, it is aligned to the original position by the tab that remains behind when you remove your cleat. My Sidis support the Look position memory feature. Yay. Also, cleats are available pretty much everywhere in this neck of the woods, so its easy to find replacements on short order.
I have read about complaints with the release tension being too light for sprinters. It hasn't been an issue for me, although I'm not one of the big boys. I did tighten mine down half way and I find I have to push down quite firmly to engage the cleat which is tighter than my setting was for the Keo Carbons. And I know I never had a problem with my Carbons at my max - about 1200 watts. So I'm certainly not worried and sprint with them as hard as my little legs can carry me.
I didn't opt for the Keo Max 2 Carbons, I just bought the Keo Max 2 pedals. The significantly greater cost wasn't worth the insignificantly lighter pedal weight (70$ for 8 grams). Otherwise the pedals are the same. By the way, I don't like the look of the Keo Blades - I can't imagine those withstanding pedal rub during a crit or a slide-out crash very well. Seems like a faulty design for hard cornering as the carbon blade is too exposed in that position. And too hard to replace. And too expensive.
Bottom line? If you like Look pedals and/or you like a stable pedalling platform while providing customizable float, the Keo Max 2 pedals are a good choice.