A PC Mag had some quality time with one of the leaked betas of the first service pack for Windows Vista. One of the things they immediately noticed is that SP1 gives Vista a noticeable performance boost, it makes the operating system much smoother and more responsive.
There are many rumours flying around as to when SP1 will be released, but there's nothing really concrete. The best that can be said is that the public beta is due later in the year, with the final release happening sometime in 2008.
The big question is whether the hint of a service pack on the horizon will encourage businesses to hold off deploying Vista, especially if they're contemplating Windows Server 2008 as well. This isn't a situation Microsoft wants, but if it's generally known that Vista SP1 has substantial advantages of non-SP1, it's difficult to make a business case for early adoption.
Personally, I haven't had too many problems with Vista to speak of, but if this private beta is anything like the final release, it can’t come soon enough. I want those performance improvements on my main desktop machines!