I did an upgrade ... then got a new hard drive (big hard drive are under $50 now) and just installed win 7 CLEAN and I am impressed at how much better my system runs and how much less disk space it takes. I had the old drive installed and could easily copy my programs and install software that I actually use. When I upgraded and ran a driver detection program, 32 hardware drivers were outdated.... when I installed clean only 4 were outdated. I have blogged my speed tweaks here
Windows 7 Forums - tblount
Unless I was installing win 7 for someone else there is no way I would install win 7 for myself except clean... when i finished the major tweaks and installed the software I wanted and copied all my old working files from the old drive. I cloned the Win 7 drive over the old hard drive, making it bootable with True Image. I can boot into either now.. just from my multi-boot menu. Old saved email was the only problem. I had to save emails as text to a folder that I copied over to my new drive. Even the new win 7system image doesn't include email... not that I need system image with a backup cloned drive.