Well, I had Vista Ultimate installed via Boot Camp, and it's sheer size (14 gb!) and bloatiness forced me to delete it (had it for about 4-5 months). I went back to installing XP; it's over 10 gb smaller, more stable, and just more efficient to use on my MBP (it also doesn't make my MBP get nearly as hot as Vista does).
I agree with most of the comments in this thread, although some points may be overstated. For example, Vista uses a different type of installer from XP, copying everything to the disk and installing from there, which is why it requires 15GB free to install. After installation it consumes about 6GB or so (6.07GB in my Boot Camp install of Vista Ultimate) for system files. You can remove unneeded files and cut that back further if you want.
As to the heat issues, I understand that it is because Vista does not control the system fans (Boot Camp support for Vista is still experimental). I would expect that to be fixed in the released version of Boot Camp, but either way there is a utility called InputRemapper that addresses the fan issue.
Performance is mostly comparable to XP *if* it has plenty of RAM. The driver model is much more complex than with XP and it's still fairly new, so the driver situation is not as good yet. Of course that doesn't matter *unless* a driver you need is not available or doesn't work well.
Security should be better with Vista, which you may want to consider when allowing Windows onto your Mac. The admin bumps can get to be a bit annoying.
An important consideration to me is that Microsoft is phasing out support for XP: no OEM installs on new machines after this year, and retail sales ending the following year. At some point you'll need Vista to run newer versions of Windows apps or just because it is being supported.
My bottom line: if you enjoy playing with new OSes, you should be fine using Vista. If you don't care about the latest stuff and just want to get work done, go with XP and move to Vista in another year or so.