I'm going to give my 2p on the matter
Like most service packs, Vista SP1 is mostly a roll-up of fixes already released
through Windows Update. Occasionally there's a more significant core change, such as with XP SP2, but that's historically less common.
You will often hear and read what appears to be an enlightened strategy of waiting until the first Service Pack is released before adopting a new OS. Fact of the matter is, the actual date of release is pretty arbitrary. If a lot of your customers were saying they wouldn't upgrade until a service pack is released, what would you do? Release a service pack.
Every release of Windows has seen areas of improvement and areas of deterioration. SP1 doesn't fix a lot of the core complaints, because most of those are not actually accidents but design decisions. These design decisions have led to decreased performance and sacrifices. The Windows XP launch was greeted with much the same sentiment. It's actually quite ironic to see people decrying Vista in favour of XP; perhaps they don't remember, or perhaps they're just too new to this game.
The moral of the story is; make the switch when it makes sense for you, not at an arbitrary date. Absolutely do not rely on Microsoft to make Vista more what you want it to be, because that will never happen.