If you're into professional recording or DJing, chances are that you've heard of the HD-25's. They are absolutely incredible headphones, and it's no wonder that they've just about become the industry standard in monitoring.
I've had mine for a very long time now, well over 5 years, and they're still just as good as they were the day I bought them. The great thing about these is that they're designed for heavy use, so everything is replacable. I've had to replace one of the foam bits on mine after it got cut somehow, and I think the cable is finally on it's way out (mine have had a very hard life!), but the parts are readily available from a lot of places if you have a search on the net.
They are expensive headphones, I think mine were around the £100 mark when I got them, but once you've got a good pair of headphones they'll last for years. Everything is designed to come apart if someone stands on the lead or they get caught on anything. The phones themselves spin round all the way which is nice for DJing, but also means that the cable gets tangled up quite easily
The sound is obviously incredible, as you'd expect at this price. The headphones have a very flat response, giving you a very neutral second source for monitoring. They don't half have some kick to them. The bass sounds more akin to that of a subwoofer than a pair of headphones! The high end is crisp and the mids are clear.
The only thing I would say against these headphones is that they have a tendancy to be a bit on the quiet side when plugged into some portable CD or MP3 players. It's fine with an Ipod but with my old Sony discman I remember I had to have it on maximum volume in order to hear anything clearly on a plane.
That's not to say that they let noise in though. They're beautifully quiet in loud surroundings (hence why they're so good for DJ's), and they're comfortable all day too. I once did an entire days worth of recording with them on, and they only just started to get uncomfortable towards the end.