Further to VHG's reply: All the reports I have heard from testers are that, while the service pack provides minor improvements in some areas, there is actually degradation of performance in others. What's more, while the service pack is unlikely to fix any problems people may be experiencing with drivers, some drivers which were working prior to the update, either fail completely afterwards, or need to be uninstalled and re-installed.
I wouldn't class any of that under the heading "that should iron out the bugs in Vista".