I installed it a couple days ago on a reasonable, but not cutting edge machine (single core a64, 1GB RAM, 6800GT AGP). In terms of stability I would say that it is more than stable enough for basic tasks, and I haven't experienced any crashes doing anything.
While stability is very good, performance is a mixed bag, at least with only 1GB of RAM. On a fresh install, first boot, not even with all the drivers installed Vista was taking up 700MB of RAM, compared to a little less than 200MB for XP SP2. Graphics performance seemed fine with nvidia's new beta, but the system is so strapped for RAM than hitching makes games really unplayable. I would never install Vista on a 512MB machine. Vista's GUI seemed to actually be more responsive than XP's, at least when it wasn't swapping, it looks good, though that 3D switching feature looks horrible and needs antialiasing which I can't figure out how to install.
Other random things that kept Vista from being usable day-to-day for me include the inability to set up 5.1 speakers. You can select it, but when you click test speakers, only the front two work. Windows ICS refuses to enable itself
Finally, Vista doesn't play nicely with RAIDs. I have a RAID0 array on two of my SATA ports and two single hard drives in the other two. I couldn't even get to the stage of installation that lets you load drivers for drive controllers until I disabled the SATA ports the RAID is connected to in the BIOS. The DVD would boot, say there was an error detecting disks and just hang. After I disabled the RAID, I then installed it to one of the single drives and after reenabled the RAID and installed drivers and I can now access the RAID, but this is not the way it is supposed to work