It's quite annoying because I rarely actually have time to play the damn thing (until yesterday I probably hadn't even switched it on in 2 months!), so in some ways I shouldn't bother replacing it (and probably wouldn't miss it too much if I didn't) but it's nice to know that it's there - you never know when you'll get a sudden urge for a Halo marathon...
There's also the library of games, like Trials, Rez, Perfect Dark, Shadow Complex, Doom 2, Limbo etc etc that I have downloaded which I'd presumably lose if I didn't buy a new 360. Also, Gears of War 3 is out this year, which is an essential purchase for me.
Hmmm. I'm doing a bit of work on the side for some bloke at the moment - if that all pans out OK then I'll use some of the cash from that to get a new 360, I think.