I picked up a Mantis X back in March and have a few thousand dry fires with it. I have had some minor issues with it not detecting shots on my P10C but these happened less with setting the sensitivity to high. I also have some issues with a shot being detected when racking the slide which are not a lot but a few times very annoying. Racing the slide pointing down helps but is not foolproof. It really sucks when you have a great run going in the benchmark drill and have it happen as they cannot be deleted.
I am finding it sensitive to transitions and not always picking up shots without time stabilizing.
Overall I like it for giving feedback, graphing movement, documenting sessions and progress and general fun comparing sessions to each other and to others in dry fire group. I think once Mantis allows for the recording of muzzle movement as Patrick suggests it will be even better for more stuff and in live fire. They have playback now with new version but I haven't shot it live fire yet since the update.