The QoS Packet Scheduler service is used for 'special services' that microsoft feel are critical.
Guess what? Windows Update.
I'm not aware that it does anything else other than provide QoS when running Automatic Updates.
It may also throttle Multicast (Video Streaming over IP) traffic, but this isn't documented.
I'm also not aware of it slowing Ethernet or ADSL connections. I have it enabled, and there has been no issues with speed over an Ethernet connection.
You may find that Windows Update will slow down your surfing/downloads and vice versa with it disabled.