Pros: Sweet design, nice pads, Great quality
Cons: Too tight around head, annoying jack, expensive
I'll try to keep this short 
I'm very impressed with this set of cans. As other reviews say: The quality is awesome. They have nice, strong bass, nice highs, and nice mids (not sure why eveyone says they're recessive. They sound fine to me, but I'm not quite yet an audiophile.
These headphones do have a downside, though (from my experience, at least). They really clamp on your head. Hopefully they will start to feel better over time, but as of now: they are tight. Next, the damn jack. That thing had me freaked out at first. First, try to just put it in your computer. See if it works fine. If it doesn't, do not worry: you just didn't jam that ******* in. On my laptop, I had to seriously push that thing in. Then it performed very well.
I can't tell you if you should purchase these, because they are my first serious cans. I think they are on the expensive side, but it's not terrible. For $150, I'm enjoying the hell out of these M50s. I definitely made the right choice.
EDIT: The clamp is easily fixed. I have been stretching them out and it worked like a charm! They fit amazingly now.
I got lucky with my purchase, though. I got them before the redesign when they were cheaper - $90 brand new on Ebay. At the new price, many audiophiles are saying you can do better than M50s.