I own the HD555s myself. Have had them for about four years and they're still going strong. Quality sound and comfortable to wear, although they do leave big imprints in your hair
Anyway, my friend said he tried out the Beats and was very disappointed. He's a bit of an audiophile so I trust his opinion, but I'm afraid I don't have one of my own.
What about modding the HD555 to HD595?http://mikebeauchamp.com/misc/sennheiser-hd-555-to-hd-595-mod/