I have one and love it.. I can monitor swing speed ( to make sure she's not slowing the bat down or up on pitch speed) path of the swing, impact zone and it will also show bat deflection if they're not driving through the ball and other useful information. Between that and having a video camera set up I can monitor every part of her swing while I'm pitching to her.
DD's hitting instructor has one and absolutely loves it. He tests the bat speed and swing path of every student that comes in with it, and then emails them the results. The one thing he said though is that the batter has to stop the swing and start from a loaded position in order for it to read correctly. This slows down the swing speed because it's not very smooth that way (hopefully that makes sense