If you're gonna play solo, get L4D2. You have the option of setting every character beside yourself as AI, and a friend can jump in at any time.
I'd have to disagree, you'll get bored of doing the same campaigns over again with bots faster than you would playing single player Borderlands. Plus playing single Borderlands at least you're leveling up your character, L4D2 would be practising. Also Borderlands has a zombie dlc campaign anyway ^^
He makes a good point, if you haven't picked up stuff like Bioshock, Fallout 3, Mass Effect 1 or 2, or GTA if you're into that, you could probably pick up two of those titles for the price of L4D2 or Borderlands (although I think Borderlands might be quite cheap pre-owned now, not sure) But if you're going to be playing on Live as well, L4D2 and Borderlands are both viable options, as if you're not playing with friends you can play public games; L4D2 being the better choice there as Borderlands tends to be difficult to find decent players online. Mainly because of people using 'modded' (hacked) guns, but as long as you're hosting you can just kick so it's not too much of a problem. As a side note, something that would give you great single and multiplayer experiences is The Orange Box, especially if you haven't played Half Life 2, HL2: Episode 1, and Episode 2. There's also Portal, which is a must play imo, and Team Fortress 2 for some delicious CTF or other tactical team games. The icing on the cake is that it's dirt cheap now, I picked my copy up recently for around $10.
I can't wait to get my 360. EDIT: I'm sorry. This post seems pointless now that it's posted. It can be deleted.
Yeah, SP is fun, but no one plays MP at all..... Literally. If you go in free-roam online, you'll find about 2 people... No way. I could name 10 better games.
Only when you suck and hit a brick wall in your ability level. Work past that and it opens up all over again. Plus the endless stream of awesome DLC is a reason to keep going back. ...and obviously Rock Band 3 will expand on that even more, with Pro mode and it's 218-button guitars, and the 24-key keyboard.