Well, they're very musical and can sound very nice. They don't have a ton of output but are very clean. The only problem is, just like any other enclosure, they need to be designed around a specific sub. In order to get the best sound out of a transmission line, you really need to tune the length of the line to the Fs of the sub you're going to use.
Buying some pre-fabricated TL is just as bad as buying a pre-fabricated ported enclosure. I would stay away.
Oh, and those enclosure would really just act like a low tuned ported enclosure. Their line length looks to be close to 50" (estimated by the exterior dimensions) which is about a 60hz line length. I would stay FAR away from those enclosures.