I've had Leopard running on my Mini 9 since I purchased it 6 months ago. When I came to a fork in the road of installing Snow Leopard or Windows 7 - I decided to give Windows a try again after jumping ship with Vista.
After a week of heavy usage, work and personal, I can say I AM GLAD I WENT WITH WINDOWS 7!
I did a regular install via a USB drive (I didn't use V-Lite). After the install I did most of the suggestions in the Windows 7 SSD
thread (Turn off System Restore, remove un-needed programs, turn off indexing and defrag, etc).
I experimented with differernt Pagefile settings and currently have my Pagefile set for the default 2GB size.
I have Office 2007 SP2 installed, Safari, and some other odds and ends.
The performance is EXCELLENT. I can watch shows on Hulu without an issue, I have all of the "pretty" features still on (like Aero peek, etc) and it all works flawlessly. I use torrents on a regular basis and my Leopard install with Transmission would choke and I would never be able to download using my full bandwidth - that isn't a problem now with Windows 7 and Utorrent. MS Office 2007 runs surprisingly well, I use Outlook all of the time and it doesn't even feel like I'm using any resources when it's running.
All and all I am extremely pleased with my Windows 7 install and would highly suggest it to everyone else. The MAC-ish Taskbar is great (I have it on the left instead of the bottom). Programs start and close very cleanly and quickly. The sleep feature works like you would expect. Dual-Monitors, clone, or a single external monitor are now options (I couldn't do that in OS X 10.5).
If you are looking at this thread wondering if you should take the dive or not - DO IT! You won't regret it.