Vertical mice have been around for a very long time. I have one, and attempted to use it for a while, but having three rigs at home, and one at work (at the time), it seemed to be an expensive experiment.
I suspect that if you give yourself the time to get used to it, and I mean lots of time, the benefits do exist.
I also think that arm position, desk/chair height play an important part.
Everyone will have different experiences so experiment and see what benefits you the most.
EDIT: In the same vein, the MS Natural keyboard (the original) is still the most comfortable keyboard that I have ever used. I don't currently use one, though I have two on the parts shelves, but it beats a straight keyboard for long term comfort. Not for gaming but pure typing comfort.