It sucks in comparison to the WMV streams for several reasons, mainly the difference in bandwidth.
Xbox streams vs. PC streams are night and day, even SD. Max on WMV was about 3400KB/sec (not sure what their "HD" streams are, but I could easily find out by watching my router's bandwidth on Tomato while streaming HD).
Internet service at or above 10Mbps is plenty for Netflix at this point. Get a dedicated device and see if you notice any difference - I would bet that you do.
actually, they restrict HD to "compatible devices" such as an Xbox 360 ...Not sure what other devices do HD, but I'm sure if you researched a specific one you could find out.
All I know is that the streams on those look much better because they still use WMV instead