Personally, I think if it ain't broke, don't fix it & I know plenty think Vista is broke out the box
Vista
was broken out of the box, well before SP1 anyway, which was a
major improvement.
There's a few improvements in SP2, but its more of a roll up of all the former hotfixes and security updates since SP1, so to most wont be such a critical update.
Still no reason not to install though, remember you can also clean out all the old SP1 and previous hotfix files which will free up a decent bit of disk space.
From a repair techs' point of view, its nice to be able to go straight to Service Pack 2 standard instead of SP1 and having to spend ages downloading all the patches from Windows Update.