I experience increase depth of sound stage with my Audio-gd Digital Interface with the new driver. People using Windows 7 will want to try it out.
Overall SP1 also make the Windows UI more responsive.
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I'm seriously shocked, gifts don't usually come this way :)
Experiencing exactly what you do.
Dang. An update to the Windows USB audio drivers is long overdue.
There are several hotfixes in SP1 that deal with USB in general that may or may not have an impact on the way your system behaves when streaming via USB audio. As best I can tell from reading the release notes, there are two audio specific fixes:
1. Some USB 2.0 audio devices that have an MIDI out port do not work correctly in Windows Vista, Windows Server 2008, Windows 7, or Windows Server 2008 R2 http://support.microsoft.com/kb/2182039
2. There is high CPU usage when using a USB audio device connected to a USB 2.0 EHCI host controller http://support.microsoft.com/kb/981214
There are also HDMI audio updates.

Dang. An update to the Windows USB audio drivers is long overdue.
There are several hotfixes in SP1 that deal with USB in general that may or may not have an impact on the way your system behaves when streaming via USB audio. As best I can tell from reading the release notes, there are two audio specific fixes:
1. Some USB 2.0 audio devices that have an MIDI out port do not work correctly in Windows Vista, Windows Server 2008, Windows 7, or Windows Server 2008 R2 http://support.microsoft.com/kb/2182039
2. There is high CPU usage when using a USB audio device connected to a USB 2.0 EHCI host controller http://support.microsoft.com/kb/981214
I've had KB981214 installed for months now, mainly because of a Windows issue with accurate CPU usage reads when using very low latency settings on my DAC. It's very good that they decided to ship that hotfix with SP1, specially since that particular one can't be downloaded through Windows Update.
EDIT: So far SP1 is running smoothly, no performance improvements here like some I've read around forums, but it certainly hasn't degraded one bit, and increased usability for me with little things.
So far, I can recommend SP1 for just about anyone. In the event something goes wrong, SP1 can always be uninstalled with full rollback.