I'm a tweener, 15mph medium revs.  When I first got this ball it was a beast.  Hooked a lot and came back when I got it to the dry anytime.

Here's the issue, the cover on mine is dead after 80-100 games.  I've rejuvenated it, I've re-surfaced it, hot bathed it, the whole nine yards.  I still can't get the same reaction I had out of box.  It still hooks, but it's at least 5-6 boards less than when I first got it.

It's the first ball I've had that OOb was AWESOME, and later is not so great.

Two reasons -

1 - The coverstock is very high maintenance becuase of the high oil absorption rate,  so you have to take the oil out of the ball frequently in order to preserve performance.  Personally, this is one of those balls that I'd use clean and dull on after every session in order to slow down the effect.  The other factor is that since the coverstock is very strong, it'll roll good out of the box but when the bowler burns a track in it, it's burning energy earlier and not turning as hard on the back as a result.  One way to minimize this is to hit it with a 4000 abralon pad once or twice a month, depending on the condition of the lane surface you're bowling on.  

2 - The core design of this ball is such that it needs negative side weight in order to work properly.  The HART is supposed to be in your ball track and traditional MB drills make this ball roll like an expensive White Dot.  If you're friendly pro shop operator doesn't know this and puts a traditional MB drill on it, you just bought a $240 spare ball.  

I haven't actually thrown a No Mercy, but I'm good friends with one of the local pro shop operators here and that's how it was explained to me.