I purchased a new machine in March with Vista Ultimate X64 preloaded. I experienced all the problems that Tony Hopkins mentioned as well as a few others.
If speed is an issue, Vista runs between 10 and 30 percent slower than XP on benchmarks I had completed.
Some hardware, such as webcams and some wireless cards/configurations have difficulties.
Between 50 and 80 percent of the applications I used had to be replaced.
Vista has a bug that can destroy a network while synchronizing with it. Although a patch was released within a few days after Vista destroyed my network, it does not matter because you have to have the patch installed to minimize this issue and you have to access a network to install the patch. In other words, the chicken and the egg.
Vista has a memory leak. When booting a clean install from a retail copy with nothing but the OS installed, the computer used 639 megs of ram. After sitting idle for two days, stand-by disabled, the ram use increased to 845 megs.
XP starts with about 25 services, Vista starts with about 45.
Something as simple as changing a hard disk can deactivate the OS.
Disabling the UAC disables RPC endpoint mapping and writing to temporary folders, which can inhibit production roll-out.
Vista has by default 7 system accounts, only three of those can be disabled.
Vista has numerous call home feature that I had to disable via a network firewall. This may or may not be a concern, but because the information was encrypted, I chose to err on the side of caution.
When SP 1 is released, I will test it again. I am not, however optimistic. I did work with, and test Vista for 6 months before 'Upgrading' to XP.
Unless you have an overriding need, I recommend against Vista.