Using Arch and loving it =)
I have used Ubuntu, it's decent. The new changes kinda suck imo. And Win 7, urgh... Just no.
The addition of Unity to Ubuntu isn't as bad as it might seem. Sure, it needs a little bit of work, and if you're the sort of person that's using Arch, the obviously Unity is going to be WAY too locked down for your usage. For most users, it's a nice change.
If you're not a windows person, then I can see why you wouldn't want to use Windows 7, but other than that, what's wrong with it? It's a little more polished that Vista and a damn sight better than XP.
Honestly, if one more person asks to buy a new computer with XP, I'm going to slap them. It's >10 years old, has horrible security, doesn't have a reasonable 64-bit implementation, doesn't support so many new features (DX10,11, etc), doesn't properly support newer hardware. And quite frankly, it's ugly. :/
Vista supports most of these, yet everyone seems to hate it. That's it. I'm making a Vista appreciation club. :D
[/rant]
For the record, I use Windows 7 Pro x64 on my Desktop, Mac OS 10.6.7 on my laptop and Ubuntu 10.10 on my fileserver.