I owned one for about 4 years. It has a nice tight and focused tone - very Mark-like. I don't know if it exactly nails the Mark III tone but I now have a Mark IV and the .50cal + had some of the same tones in it.
The only problem I had with the .50cal + was that going from a great clean to a great lead tone was near impossible on the fly. You can have both, but you can't really have them without twisting some knobs.
Hope that helps. If I ever find another .50cal + for a good price I will buy it because it would be a great compliment to my Mark IV in a stereo setup.
