I recently bought the ATH M50 and at least 8 other headphones over the last 2 months. I've been auditioning (i.e. buying and returning) some of them, other's I bought 2nd hand and have listed them here or eBay as I move through them.
My reasoning went like this: I can ask questions until I'm blue in the face, or I can simply experience them.
So I set aside $1,000 or so and went nuts ordering headphones (rather, my credit card has been getting slammed). I'll get almost all of it back, minus the cost of the pair that "wins" and also misc shipping expense that aren't recoverable.
So, what did I learn about the M50 specifically? It's worth the hype. But the key is to understand the hype. I suspect many here believe as I once did that the M50 is delivering $300 quality headphones for $100-$150ish. If you buy your headphones from Best Buy, this is true. The ATH M50s destroy any and all Beats. I'll stand by that. They destroy most of what you find in retail stores, because retail stores are overpriced and sell more based on visual aesthetics rather than sound quality.
But, in the head-fi world, the M50s are a good value, but you're not truly getting more than what you're paying for. I got mine for $110 and they're worth every penny. I've paid more than that for headphones that should have cost less.
Now, about the sound: they're great and nothing has been able to beat the sub-bass they generated when amped by a Digizoid. Nothing. The only true basshead can I tried next to the M50 was the Crossfade LP1 and it couldn't go as low as the M50. It was louder, sure, but muddy, uncontrolled, and just couldn't hit those lower Hz ranges.
The mids are recessed, but it's not that big a deal.
The highs are artificially boosted, but not normally sibilant except in the incredibly rare exception of a few poorly mastered albums. Check out Thom Yorke's album "Atoms for Peace". If your highs are pronounced, this album will reveal them. Don't worry, it's not annoying. Fatigue will take hours to set in.
Overall, the reason I'm not sticking with them is sound stage. They're completely 2D. Not claustrophobic, but as close as you can get to claustrophobic. Sound stage isn't something I learned to appreciate until I had gotten a good 50 hours of critical listening. IMHO, this is the achilles heel of the ATH M50. The unnatural sounding highs and lows are slight weaknesses, not glaring flaws, but the flattened sound stage most certainly is.
In summary, they're a great reference point, they're certainly worth the price of admission. Maybe you'll be one of the lucky ones and just be content with your purchase. Me? I'm a lost cause. I've become hopelessly addicted. My current favorite is the A900x and a couple times a week I'll hop online and scout a replacement in this futile, endless search for perfection. Maybe the M50 should be reclassified as an aural gateway drug.
The W5000 is one of my all time favourites too. For me: W5000 > L3000 > W3000ANV > everything else...