I have a Tailsock on my Nimbus, as shown in
this thread. I bought it as a closeout from Terracycle. I wanted to try it out, and I figured it would also make a nice pattern to follow if I get around to making a full bodysock for the Nimbus, like I've been thinking about doing. I wasn't that keen on the frame setup that everyone seems to use with these, so I built a coroplast box to fill it, instead.
How much good these do is a hotly debated topic. They definitely do make you faster, but I don't think it's nearly as much as Hostel Shoppe and some of the other places claim. From the studying I've done, and my own experience, I think it takes something a lot longer, with a lot more gradual taper to make that kind of difference. I think a 1 mph gain would be closer to what you could expect, depending on the bike.
On a lwb, I don't think they help much at all, unless you also add a front fairing. They really do help with your visibility, and also have a high cool or geek factor, depending on your point of view. YMMV.