I have a pair of Sennheister 280 HD Pros - they've lasted me about 7 years, an excellent set of headphones. I used it to help critique music for lots of artists on their production, and I know lots of artists who use it as a cheap pair of mixing headphones.
Work bought me a pair of 380 HD Pros, and I'm impressed on how much of an upgrade they are over the 280s - I can only imagine how good the other Sennheisers are.
The 280s are closed back. Great for isolation, for not letting ambient sound interfere with the music. It also changes the way the transducers work, a little bit.
The 600s are open back. Obviously there's no isolation, but the transducers work more freely. It's a bit easier to distinguish tiny sounds from a huge background.
I've both the 600 and the 280. Great phones both, in different ways.
Work bought me a pair of 380 HD Pros, and I'm impressed on how much of an upgrade they are over the 280s - I can only imagine how good the other Sennheisers are.