My main OS would be Winxp since it came with the rig, and i have to say something that comes along with a package does last long and is better compared to those downloaded from net, i downloaded Winxp image from my Technet account, and that image i made it to oem and after a few weeks of heavy browsing and downloading it would not behave the same as the original copy that came with the rig.
That wouldn't give any reason to prefer more than Win7 but, i really like the eye candy and the whole aesthetic feel of Win7 when i simply leave it open, i don't know if its me, but i noticed the copy/paste feature is considerably faster in Win7, and tweaking command prompt wasn't to time consuming, and the major thing i liked was configuring the recording option and selecting main audio device, which was replaced by some sound card was also easy, all these small things have been improved.
I am an average joe, and i do not use my pc for any major things, but i tweak a lot and noticed a lot of the small things i had a little time consumed in Winxp has been reduced in Win7. But i will not leave Winxp that easily or any time sooner. I can't comment on kernel level or can't give any comments what an administrator found it useful to him/her.
I am on a dual boot, and it is rocking for now.