GR Research Subwoofer

I came across the GR research subwoofer / PR combination the other day and am interested in building one for a friend.
Has any one built this yet? Its on sale right now for $130 for both the PR and the subwoofer.
It seems like a good deal, how does it sound?
The single 12" PR has me a little worried, is that going to be enough displacement?
I'm thinking if I do it to partner it up with the Rythmik 380 Watt amp.