Mambwe,
Where did you read bad reviews of Speedplay?
The only negative I've read is about the standing
on an icecube feeling, and that only takes time
to get adjusted to.
Seriously, Speedplay are really good for bad knees
(I know I got 2 bad knees), For mountain bike I'm
guessing you're talking Frogs here?
I have x series and love em
no problem with knees, and
about 3 rides to adjust to the ice cube thing.
of course YMMV.

Marty