I have installed SP1 on my Dell Lattitude D820 with Vista Ultimate x64 on it. It is quite a bit more snappy in the internet browsing, and the OS seems to not be so sluggish.
I don’t have any more than that as I just installed it, but thought I would share that the install took about 45 minutes to apply, and all is well so far. I will keep my blog up to date with any things that I notice are different.