I'm a sound artist and field recordist, and I use these headphones both in my studio and in the field.
The sound is what you expect and love from Sennheiser headphones: clear, neutral and full of detail. On top of this, these cans have a lot of isolation, minimising bleed from outside so you can focus on the sound you want to listen to, even at lower volumes (and hence save your ears). Especially in the field this isolation is very welcome. It also makes them good tracking headphones.
The over ear design makes them very comfortable to wear, even for longer periods of time (hours in a row).
My one gripe with these headphones is the cable. They have a little bit of coil to minimise contact noises, and for the rest it's just a normal cable of adequate lenght. However, I found that this design doesn't allow me the flexibility of a fully coiled cable, and when used in the field, the cable (and the small bit of velcro you can use to wrap up the cable) quickly gets in the way of many of your movements. And it doesn't really diminish the contact noises that much. It's for this reason that lately I've left this headphone at home, and choose to use my HD280 instead. It has a very similar sound and practically just as much isolation, but the coiled cable of the HD280 makes it just a lot easier to use (short connection for most of the time, but can stretch to quite a distance when needed). While the cable of the 300-pro is replaceable, unfortunately Sennheiser does not make a fully coiled cable for this headphone.