I have purchased WT+ on BA a couple of times. Both times, I paid to upgrade a WT ticket at the airport.

In terms of cost, I found this to be a little more reasonable than buying the WT+ tickets outright. I'm not particularly tall at 5'8" or large at 160 lbs but I do enjoy the slightly wider seat and the smaller cabin. I wouldn't pay much more than a few hundred dollars for the product, though. To me, the biggest benefit of any kind of upgrade is seat width to get a little more elbow room and to avoid having to fight over armrests. The greater the width, the more likely I'm willing to pay more money.

I agree with the other poster that mentioned this cabin appealing more to business travelers who have policies forbidding company-paid J travel. It doesn't make much fiscal sense otherwise.