Can also buy it digitally for $25 on PSN/Xbox stores, although Gamestop is offering $12 trade in value which is higher than I expected. Still, its a 70+ hour game for most people so its not like you are going to beat it in a weekend and then trade it in.
I play a few hours a week, switched from physical to digital when it went on sale a few weeks ago (for $25) and its nice not having to eject the disc. I did buy the DLC at gamestop though for $10, you get a digital code inside and a deck of cards which I haven't even opened.
If you are on PSN and switch from physical to digital you have to redownload the game but keep your saves, on xbox you keep your install when you switch.