I am currently using Windows XP Pro x64 Edition. I'm kind of a die-hard XP user, and XP x64 gave me nice things like 64-bit address space, Storport storage driver model, and GPT data volume support for my large RAID-6 , so it's features which I all use, that's kind of the maximum I can get out of XP.
I even use it with quite modern hardware, i7 980X, 24GB RAM, an Intel 320 SSD with my system partition properly aligned to 1MiB (and TRIM on XP using the Intel SSD toolbox) and even two GTX 680, because I need a lot of rendering power for my 2560x1600 resolution.
It's really astounding what kind of hardware still runs perfectly fine with XP x64, like my Logitech G25 racing wheel, the X58 chipset, X-Fi, GTX 680, SSD, 3ware 11TB RAID-6 and so on.
Of course XP is dying, there's no denying that. But I'm going to use it as long as I possibly can, since I just love the OS (as much as I love Debian Squeeze).
By the way, does Rebellion run on XP?