I have a lot of AudioTechnica headphones. I love their designs, but I've learned one hard lesson about them: lots and lots of hype surround these headphones. Which is why I ended up owning so many.
AD500, AD700, A900X, M50 & ESW9A are the ones I went with. So far, only two of them are worth having. The others were basically just duds in my opinion, over hyped, crippled, or otherwise meh sounding headphones for the cost
AD500 - If you can get it used or on a serious sale, sure, it's ok. I got mine for $30. Yea, it was a crazy deal. I wouldn't buy it for more than $50 though. It's an ok headphone, nothing special about it, it has a nice sound stage, but it's otherwise a grainy headphone with some sibilance and not much low end. Basically a typical headphone, nothing special, nothing really different, just very light weight, doesn't fit that well due to it floating on your head, and very mediocre sound, to down right below average in some areas. But for it's cost I was not expecting the world.
AD700 - Pretty much the most hyped up headphone there is, I dislike this headphone more than any other AudioTechnica. It has zero sub-bass. That's crippled to me. It rolls off it's bass at lower frequencies very quickly. It has nice mids and good treble and a nice sound stage, but that's all it has going for it. For the price of $80~120, it just is over priced to me. Regardless of price, a headphone should be able to play all the frequency range, otherwise, I call it crippled. The AD700 is crippled. It some how got to this magical level of hyper-internet-copy-pasta where it shows up in any thread where gaming is mentioned without much of a reason. I find it pretty much meh for music, out right now worth listening to, unless you actually dislike bass, in which case, maybe this headphone is for you. It has good qualities in the mids and sound stage, but because the rest of it is so lack luster, it just detracts from the unit as a whole. I don't recommend this headphone to hardly anyone except those who strictly claim "I don't want much bass, not even neutral" as that's the only niche it fits into. I don't even like it for gaming. I'd rather use a headphone that is not crippled, and cheaper. You can always equalize the bass out instead of buying a crippled headphone that you cannot make sound better in the low end. AD700 is over hyped, over recommended, and not the best gaming headphone for $100 or less.
A900X - A new one to join the Art series, and man, it delivered. Great mids that you would expect from AT, great treble, and that mysterious thing that the Art series lacks: bass. The A900X is bassy, it has a mid-bass hump, but is capable with sub-bass. This thing can drop for dub step. It can play trance great. It has easily bested other mid-tier headphones on my head for jpop even in a recent show down that I did. It's a fantastic headphone and to me is better than the Shure Shure SRH940 for vocals which is the only thing I think the SRH940 is worth listening to for. The A900X completely replaces that. It's superb for vocals, yet also very musical in all frequencies and is competent for all genres. It's not crippled, it's energetic, musical, and just a really good headphone to listen to. It's only fault has been comfort, it's not as comfortable as the other Art series because the pads are pretty stiff and the unit is heavier than normal Art series, so it's less light weight. The other fault, though minor, is that it doesn't have the big 3D sound stage of the other Art series that are open, but this is a closed headphone, so I expected that. It has a good sound stage mind you (big cups, angled drivers supporting it), but it's not as wide open as the open-back versions. I see the A900X as a winner from AudioTechnica. Great headphone for someone looking for a complete headphone, closed back, good for everything, that can actually drop bass, or be a quiet intimate acoustic session. It's fantastic.
M50 - The M50 is pretty much the most over-recommend, over-hyped, and mediocre headphone I've encountered. I bought one, just like everyone else, because it was supposed to just be so good. Well, the best thing about it was that it was built like a tank with 180 degree swivel cups, instead of just 90 degree and that it was relatively isolating. So it was an excellent beater headphone for traveling. But for music? Not good. Just mediocre. Sounded like a typical closed headphone to me. It gets heralded as being flat and great for audiophilia. I don't think so though. It's bass is bloated, its mids are cold and a little recessed and the treble was a bit harsh. The sound stage was small, terribly small, it felt stereo. And it just wasn't that comfortable. Can it drop bass? Sure it can. But the rest of the headphone is just not up to par in my opinion. It does bass well. But the rest of the headphone is just either mediocre or not pleasant to warrant it's price tag. For $60~80 this headphone would make sense. But at $150~160 it doesn't make sense to me. I generally don't recommend this headphone to anyone unless they want an absolute tank-like headphone that can survive as a big sturdy portable, it does that job fine, but for casual listening? No thanks. Unfortunately this headphone makes it into virtually any thread because it has become the most widely copy-pasta recommendation in all of headphones. It had a short summer of super hype, and it's waves have not finished hitting the shore line.
ESW9A - Beautiful on-ear classy woodback headphone with lambskin, very luxurious. Comfortable to wear, even for on-ear. The sound? Gorgeous. One of my favorite AT's. It's treble is gorgeous, so smooth, detailed, but not shrieking at all. The mids are like butter, vocals are great, instrumentation is great, and it's not recessed. The bass is fantastic, it doesn't bleed anything out, has just enough impact and oumph to satisfy a basshead, and drops very low too, it's bassier than neutral so it immediately gives you a sense of warmth, organic, rich, full sound. Sound stage was not superb, but it was rather normal for this style of headphone. If this headphone was a fullsize instead of on-ear it would be nearly perfect and compete with Denon in my heart almost. Lovely headphone. I enjoy it more than the Vmoda M80. They actually sound very similar. But I found the round on-ear approach better than the Vmoda approach.
So that's just my experience so far. I really like the A900X and the ESW9A. They're great headphones. The rest, well, there are other headphone companies thankfully.