I agree with omegastrada in that it is all down to personal experience and preferences.
Montello, you know I'm only messing with you

I've ridden a couple of Pinarello but never owned one, I really liked them and they rode great.
Stringfellow I've also ridden a few De Rosa as well, not sure whether the steel ones were SLX or not, but all rode really nicely as would be expected from de Rosa but no better than my Merckx and certainly there was one non steel machine that was very muched liked.
I was really just speaking from my main experience or long term experience with my Merckx, theres got to be a very lot of people out there who have built with SLX, I imagine some good and some quite bad.
I wouldn't think that you could go too wrong trying any of the frames from the main known builders.
Choice is a wonderful thing and this forum is great for this as you can get a good cross section of peoples experience.
Jamie
Jamie