I opted for the 64-bit version since I have a 64-bit processor.
The install process was incredibly easy. Since I bought a standalone version it did not require a former version of Windows.
It automatically detected most of my hardware and drivers were easy to find and install. I have had no hardware issues at all.
The new UI is very good looking and easy to navigate. The widgets on the sidebar are a very welcome addition.
UAC is very annoying so I disabled it. Other than that, I have not found any problems with this OS. My programs all run (some require compatability modes but luckily this feature actually works in Vista!) just as well as they did in XP. I haven't had any stability problems and quite frankly, I have trouble finding things to complain about like everyone else has. It's a solid upgrade from XP and I enjoy it.