I make soundtracks for videogames, and wanted a headphone good enough to mix my projects with when I'm not at home and to use as a second reference for my monitors when I am. A friend of mine gave me the best recommendations about the Sennheiser HD 650, let me borrow it for a couple days and I was totally convinced after that.
It comes in a big and nice box with foam enough to keep it safe in case of falling or being hit by anything (you never know). The headphone's grey color is discrete and good looking, it is easy to adjust to your head and is very, very comfortable to use, even for long periods of work: that's because the phone's headband padding quite cleverly has a space in the middle, so it never presses the top your head. Also, its earpads are pretty big and stay around your ears, not agaisnt them. It's and open-back model, so the sound isolation is not so great, but good enough anyway.
The sound quality of this headphone is outstanding. The bass is full and round, mids and highs are very clear, transient response is excellent. You can totally trust it if you want your mixes to translate well to other systems: stereo, tv, car, etc.
It might be only my impression, but to me it sounds like the stereo image is a little bit narrower than some other ones I've used. Nothing that will make your work any worse - I don't even think that this is bad, to be honest - but it might be something you'll notice in an A/B comparison.
The only thing I don't really like about it is that the cable is way too long... it might be a good thing maybe for a studio, but in my case I just don't need it.
So, lastly, I can say that the HD650 is definitely not cheap, but absolutely worth it. I would buy another one without thinking twice.