Cons:Plastic may break if you run it over with your car :).
I fell in love with these cans at first sight when I gave them a shot at a high-end music store in Europe last year, and finally my wife showed some mercy and decided to buy them for me as a birthday present. You can't go wrong with HD590, just don't think twice if you can spend a couple of hundred on great headphones.
First, they are extremely comfortable, just cupping your ears and distributing (a very small) pressure evenly over your head. You can forget that you're wearing them. This is in part due to mostly plastic construction, which at first may look flimsy, but very soon you realize it's engineered very thoughtfully and won't break all that easily.
And the sound. Just one word: real. A few times I had to turn around and check who is there in my office talking so funny, just to realize that it's the voice in my headphones. Bass, highs, mids for that matter, it's precise, you are simply "there".
If you hesitated before, just forget there're other cans out there. This is the best thing for the money.