I haven't benchmarked it, but subjectively speaking, the boot time is greatly improved, applications launch faster, and file operations are speedier, with the exception of opening an image in Windows Photo Viewer, which for some reason takes 2-3 seconds.
A lot of the perceived speed comes from barely-noticeable tweaks to the UI. Win7 does benchmark a bit faster than Vista, but it really is the UI (window response/refresh times, etc) that makes a world of difference.
