If you ignore the instructions (like we say WITH ANY KIT,) and use the proper techniques covered throughout this forum, good sanitization, temp control, using hydrometer, pitching enough yeast, having patience. A mr beer kit can make good beer. There's been at least tone member on here who submitted a properly brewed, mr beer beer into a contest, and medalled.
A mr beer keg is just another fermenter, and the kits are no better or worse than any other extract kit you can make good beer, or crappy beer, but not because it's a mr beer, because of the brewer. Like someone mentioned the MR beer sticky is a fantastic resource, as is the rest of the forum. Brewing is brewing, the same rules, tips, tricks etc apply whether it's an all grain batch brewed on a 10,000 shiny automated system, or the humble mr beer keg. It's ALL about the brewer's care and technique.