I have upgraded two and fresh installed one.
All run fine apart from a couple of driver issues (easily sorted once the vendor supplies the update).
New PC's will have the updated drivers already installed but most people will install their favourite programs and may well find they don't work properly until they too update the appropriate files.
But, as I say, this is easily done.
I've had no problems with networking but there is one thing to remember.
If you have a Vista PC but others on your network are XP, there is a special download from Microsoft's site that upgrades XP and enables it to network with Vista PC's. It's called 'LLTD msxp' and, once installed on any XP computers, networking runs like a dream.
You an always use a program like 'Network Magic' to help you. The latest version works well with Vista.