I traded in my 360 during the $100 promo.
...then I was like: WTF did I do? Now I own another one :/
I was close to doing that when it was like $175, or whatever the last really high offer was from Gamestop. I will definitely keep one of this gens consoles, but I felt like between the PS3 and 360 I needed to eliminate one of them.
I was actually feverishly trying to finish up my 360 backlog just to trade it in. I didn't make it, now my 360 backlog is bigger.
Honestly though, if I see an offer again where I can actually get MORE than the console is worth. I think it will be time to liquidate and make room for new consoles (Wii-U, PS4). Rarely have I regretted parting ways with an old console - once I actually do it.