I started using one this season, and I'm a big fan. As someone who previously used an Anser-style putter with a thin, traditional grip, it was a big change. When one of my friends on this site first saw it, he said something like, "I'd quit the game before I putt with that thing!"
Using the LAB putter did cause me to change my technique somewhat. Instead of fighting the torque like you do with a traditional putter, your goal with the LAB putter is to take out as much torque as possible. The new technique and the heavier weight meant that it took me a while to get my feel back to where it was with my old putter, but I think I'm a better putter with the LAB putter than I ever was before.
In case it's relevant, I'm a scratch golfer, and putting has always been one of the stronger parts of my game. In my view, the biggest advantage with the LAB putter is that my variance is lower. The good putting days are no better than before, but the bad putting days are fewer and less bad.
If anyone is considering buying one, I'd be happy to talk further. I get no commission, but I'm happy to offer any insights that I have. Also, just to clarify, the putters start at $400, and I suspect the add-ons like the fancy shafts aren't worthwhile. $400 is certainly a lot to pay for a putter, but that's the starting price for a Scotty Cameron, so it's not outrageous for a premium putter. When you understand the technology and the fact that these have to be hand-balanced to fit your lie angle, it makes sense that they are as expensive as they are.