I have been using it for the last couple of days (taking advantage of their 7 day free trial). It is somewhat hit or miss whether you get the Super HD servers. Sometimes I do sometimes I don't.
One way to check is if you are able to play the 'Example Short 23.976' then you are connected to the SuperHD servers. You can see the streaming rate within the title and should ramp up to 5800 kbps if you have the bandwidth.
Another way to check is if your router is connecting to nflxvideo.net domain then you are connected to the Super HD servers.
When I do get Super HD it looks good on real titles. If this was reliable it might be worth 4.99 a month to get Super HD with Comcast - but for now (at least in my experience) it only works some of the time.
We've been discussing the use of Unblock-US for getting Netflix Super HD video encodes in a thread at AVS Forum starting with this post. I've come up with a procedure which works for getting them with any 1080p-capable Netflix player that I have and posted it here. It's a PITA but do-able by the hardcore and determined .