I think that VisTablet model is the one that's flexible. I have a friend who owns one. He says it's great for travel because it's very lightweight, thin and flexible. The pen is a bit unwieldy, though, due to the battery. It has quite a large diameter, which makes it uncomfortable to use for very long, and it's not balanced quite right, in my opinion.

You'll also need to support it during use, since it is not rigid. I commonly put my Wacom in my lap, but that wouldn't work for the VisTablet without a book or a board or something beneath it.

My friend has no complaints about the quality of output, though. He opted to take it with him on a trip instead of his Wacom, and he's a professional visual effects artist, so it must at least be adequate to whatever he's using it for at the moment (After Effects, most likely).