Had a USE Alien on the back of the old hardtail tandem. Worked ok, just watch the seat clamp. The idea is good, two clamps sliding on a curved rail to offer infinite adjustment, but its held together with a wee bolt with a 3mm allen head. On more occasions that Sandy wishes to remember she had the seat tip up, and once it came right off. Not nice while steaming down Falla Brae at GT. Ended up using a long allen key and getting it so tight I was scared of stripping the threads. Checked it was super tight before every ride, then no more problems.
It worried Sandy so much (understandable, a seat post up the bum is not much fun) that we had to buy the the full suss Ventana so she could do away with it!
Rob