They have a really good reputation, but a lot of people would argue they have become overrated. I decided to get them because my old JVC Harx900s broke and they were supposedly a good copy of some other Audio Technica headphones so thought I may as well try the M50s. In the end they are still pretty much "entry level" headphones. I like them but I wouldn't call myself an audiophile.
Definitely worth getting a sound card if you have spent any money on headphones. For entry level headphones the ASUS Xonar DG is a pretty good sound card (providing you don't use Linux), has inbuilt headphone amplifier and costs like $30. Anything better than that would be around $200 or require a stand alone amplifier.
Oh and when i say "m50s" I am just referring to them in a plural form, I have the coiled cable version as opposed to the straight cabled "M50S"