I've had one now (B to Bb) for just about 10 years. It took a while for me to get used to it, but it's really a cool thing.
The one real disadvantage is that it's impossible to chime while using it. A second relatively minor problem is that things get very cramped when you're playing way up the neck. You can still do it, but it's not real comfortable.
But the advantage more than compensates for this. On a Universal, the B to Bb change is useful in so many places on both sides of the tuning, that there really is no one place to put it where it can be utilized to it's fullest musical advantage. By putting it on a wrist lever, you can always use it, regardless of what your feet and knees are doing.
I was skeptical myself until I tried one out. But once I did, I immediately saw the advantages of having such a lever. Recently I was surprised and pleased to learn that my long time friend and occasional mentor Blackie Taylor had one of these levers installed on his guitar. He tried mine out and liked it so much he got one for himself.
I'm not sure whether or not there is any advantage to having one of these levers on a single or double 10, but they work really well on a U-12.
