I've got some Sennheiser HD-590's. I've used them for the past 2 or 3 years. The big problem I have with these headphones is that they make everything sound so good it makes all my other stuff sound rubbish (if that makes sense).

I plug them directly into the output of my Creative Audigy2, sounds great (not set to headphone output mode).

I can wear them for hours and hours and never notice they are there. I have them on now, have had for the past 4 hours or so, my ears hardly touch the phones at all, they are very light and comfortable.

I've often wondered if a headphone amp would be worth it, but can never bring myself to part with the cash.

Good luck with your choice, can't go wrong with Sennheiser.

Cheers

Cris.