My main OS on my laptop is LMDE 64 bit with an Openbox/GNOME2 desktop that i've carefully tweaked the last few months. It works perfectly for me. I'm following the future of Cinnamon, but for now i love the lightness of Openbox.
The second OS is Win7 64 bit (it came installed on the laptop. it could just as easily be WinXP) which i only use to sync my iPod with iTunes. As soon as i can decently hook up my ipod in LMDE and sync it, i'll be the first one to ditch Windows altogether.
I recently handed over a desktop computer with WinXP to my mom because i don't game anymore.
I agree with barryp
. I'm cautious about the performance effects of LMDE UP4 and Cinnamon. But i have to admit the future for LMDE (at least for me) looks very promising.