Depends. Do you have any friends who also own the game? If so, get it for the same console they do - co-op is a blast, though a bit less of one at 300 ping. If not, wait for a sale - it's a great game, but there's some obvious creases. In the first fifteen minutes, whether it's the good parts or the bad that stick out to you more will make or break your overall experience with the game. Personally, I love it despite having no friends, but that's just me.
Or if you can wait a few months, Borderlands 2 (by most accounts I've heard so far) is a heck of a lot better than Borderlands 1 - particularly in the area of PC porting, if you plan to play it on PC. Borderlands was a pretty obvious console port - bad (but still mostly usable) menus, FoV fixed at an annoyingly small number...