I've got an ART MP and I like it.
To answer your question, I think that most complaints about the MP are based on the overhyped marketing shenanigans of a lot of companies that claim that anything with a tube in it sounds better than solid state gear. Also, the total gearheads will tell you that not much voltage hits the tube so it doesn't really do anything to the sound, which harkens back to the marketing thing. Why put a tube in something if it doesn't do anything?...I don't know.
Another thing that people seem to complain about is the quality control on cheaper products. The product isn't consistent from unit to unit and so forth. I bought an ART MP and after 2 weeks it stopped working. The lights came on for a few seconds and then they faded out. I called ART and sent it to them for them to check out and they said that my power supply plugin cable was faulty. They sent me a new unit though. Different pre and different plugin both.
If you've got a Mackie and like the sound of it, then stick with the Mackie pres. The ART just gives you a slightly different flavor...as any other preamp would.
Hope I helped.