The only thing that would convince me to buy one is support for lots of FPS games, as it's sure to beat a standard controller for that.
I think this will be the best motion control this gen, from the videos I've seen of it the tracking is extremely accurate, almost 1:1. If you have a look at this video
and watch for the sword spin at around 2:38, pretty impressive tracking.
But, as I've learnt from Wii games, 90% of motion control is useless and is not an improvement over simple button presses like we have now.