I bought one of these from Glenn in February, but then left for a month long business trip to Korea before it was delivered. I finally had a chance to try it out today and I really like it. It pulls the whole bass a couple of inches to the right, so I don't have to reach so far to reach the end of the neck, basically it is in the same position as it is on basses with an upper horn. That makes it a lot more comfortable to play, and it doesn't hurt my back. I'll play the Thunderbird more often now.
I had been looking for a solution for this problem for a while. I've seen some basses on TB with an extension on the strap button (with the button on top like an Epi or a 70s Gibson) but that didn't seem like a good way to do it. This is such a simple and elegant solution.