The way I'd describe the MM90's is that they have a 'bite' to them.
They sound really, really good! You can get some really incredible tone from those pickups that you can't quite get with other pickups.
That's my 2 cents, anyway. I did have a chance to play an Albert Lee with SSS and another with MM90's at the Open House. So that's where my opinion was formed.
I have an AL MM90, and have had a lot of different strats through the years. The MM90s have the girth of a humbucker, with the openess of a single coil. They sound single coilish when played clean, and humbucker-like when overdriven. This is the individual pups, not the notch positions. The notch positions are a lot fatter sounding than the notch positions on a standard strat, and can get a little muddy (in a good way). A good comparison would be the difference between an Axis and an Axis Rosewood.