I think Mega Absol is best suited on a dedicated stall team as a late game cleaner and insurance against most hazard setters. After some phazing with hazards, there's not a lot it can't clean up on, and its immunity to status helps bait attacks for Sucker Punch, as well as keeping hazards from being set while it's on the field. Virtual status immunity helps too. I see baiting a dark type attack with something like Jellicent, switching Justified Absol in to get a free +1 to that lovely attack, then mega evolving and killing it back. Definitely not meant as a sweeper, though. Sure, it gets the strongest priority in the game barring Arceus' Extremespeed, but it's not going to stop things like Azumarill, Scizor or Mega Lucario from killing you dead.
I think his main weakness, aside from being about as sturdy as a wet tissue, is four moveslot syndrome. Megabsol gets some great options like wish, baton pass, and snatch to round out his offenses, but the need to have working coverage sees to it that it'll only ever carry one of these, if at all. If you carry swords dance, you then have to fight between Play Rough, Psycho Cut or Superpower. Play Rough covers everything Psycho Cut would, but with the imminent popularity of Assault Vest Ttar I think Superpower is a necessity. It helps to smack Ferrothorn around too, but you might be better off with a mixed set, since Fire Blast is a guaranteed OHKO.
Snatch is amazing for added oomph against Mega Gengar, though. Predict correctly and nab his sub and you're guaranteed a KO or forced switch, as well as added bulk for a pokemon that really can't take a hit. A free swords dance never hurts, either.
I think I might use this guy on a team with copycat-roaring Riolu for maximum trolling power.