I have been testing Windows Vista SP1 for many months now, since the early beta versions of the update. While I can’t say that it has solved all of my problems (see My Struggles with Windows Vista for details), it has made a few noticeable improvements in performance. However, with all the media coverage I have seen thus far, people tend to focus on one or two of the hundreds of changes included in the Service Pack. The information ranges from simple restatements of Microsoft press releases to "benchmarks" which use dubious measurement methods.
Fortunately, Microsoft has recently published a comprehensive guide that lists all of the changes introduced by SP1. You can download Notable changes in Windows Vista SP1 in PDF or XPS format. The guide is long, but it provides a concise, descriptive and (presumably) complete list of what you can expect. Certainly, there’s a lot of ambiguity based on "certain types of devices" and similar marketing-friendly terms. (Sometimes I wish Microsoft would just identify the hardware manufacturers that don’t follow the rules for driver development.) Overall, it’s a handy reference that I recommend. The document also includes a whopping 35-page index of all of the hotfixes and security updates that are included with the SP1 update.
Unfortunately, even with the latest updates, I still can’t get Sleep mode to work on two of my desktop computers (one of which shipped with Windows Vista and the other which is based on a clean installation). So far, it looks like Windows Vista SP1 will be "too little, too late" for me. I just hope something happens to prove me wrong.