Last fall right after 7's release(retail) I had Vista activated and installed both service packs while it was on a VM! Originally I build this case to run Vista but only ran the RC1 for about a month and waited for the 1/31/07 release date for that version.
It typically ran smooth as silk having only seen one bad reinstall during a dual boot setup in all that time. A few things I needed wouldn't run on Vista but do run on the 32bit 7! That shows another improvement seen. But Vista certainly was a relief from the previous version which tended to see regular BSODs! I spend most of the time still solving other people's XP problems! having advised them to upgrade often.
In 2.5yrs. with Vista I maybe saw one or two Blue screens while XP In six months I would lose track! Vista and certainly 7 have already shown themselves to be far more stable while every new version still needs some fixes gradually over time. The 64bit RC ran here for 6 months straight even on a drive with 85% fragmentation and never ran into any performance loss or major headaches. 7 is still an improved version over Vista whie Vista did fare well when run here!