[:)]I might get a little carried away sometimes...But this could help you become a better bow shooter. It tells you if you tilt your bow any direction before your shot or after so you know how to adjust to make a better shot.
My advice would be to go to the local archery shop you are giving business to and ask them to observe your form. Probably won't cost you a thing and you'll get spot on instant advice. A machine may tell you your form is off, but it won't tell you how to compensate for it properly.
Just a quick story to prove my point. I just recently purchased a new bow..and it had been some time since I'd done that. The place I purchased it from I chose specifically because of the service before, during, and after the sale. Once I purchased my bow the dealer/ local pro had me shoot it with him there to not only sight it in, but to assess my form and then finally paper tune it once my form was good. He did this for 3 seperate sessions at his range, pointing out not only where my form was incorrect, but what exactly I needed to do to correct it, as well as some additional tips and tricks to compensate in areas that were specific to my body. My groups have improved dramatically in just two sessions and by the third he's pretty much assured me I will robin hood a shot or two (came close on the first lesson). The robin hood part I could care less, although he makes this promise to everyone who purchases a bow from him and I've seen it firsthand twice with the two new customers who were with me. More to the point is in over 15 years of Archery shooting/hunting, I've never had my form critiqued (except by friends) by a pro and boy was it an experience and one I'm grateful for.
Save your money, go support your local pro shop, you won't be sorry...the mark-up you pay for in products at his shop is more than warranted when you get the benefit of their expertise.