What I don't like about my 15/6:
Weight/size - mine came from a speakerhardware kit, and fully assembled its about 60 pounds. Usually 50 is the number thrown around for the weight, and 60 doesn't seem like much more, but after a few flights of stairs it does to me, especially because the cabinet is kinda big and awkward to carry.
Mid range driver is loud - its not as harsh as a tweeter that's for sure, but the mid range driver is still very snappy and reproduces a lot of content that is usually lost in other cabinets (what would be between the speakers and tweeter) and I'm not used to it. For now turning down the high EQ on my MB800 helps deal with it, but eventually I want to add some padding to the crossover.
Once I move some gear I'm gonna look into having a new cabinet built to house the fearful guts, I'd like to have one closer to 40lbs. Possibly with slightly different dimensions to fit in my car better. Failing that, I would also consider one of the lightweight 215s that have been popping up.