Hi, you can look at a G-tech meter at
http://www.gtechpro.com/http://www.gtechpro.com/
and if you click the "test drive it" tab you can watch it in action at a drag strip calculating an identical time to that of the drag strip timing gear. Which is impressive, but obviously its not gonna be 100% accurate all the time but from what I see with mine its as good as your gonna get without spending loads!! I got an older version of the one on the website though off ebay for £50 I think.
As for my B&M, gear changes are slighty quicker, again obviously its not going to cut off loads of time but the whole feel of changing is a lot more solid. It slots into each gear with a nice solid feel and its nice being able to change gear with just your wrist movement. It feels horrid driving another car with the normal gear change now, real sloppy. I bought mine a k-tec (not the cheapest but I was there at the time having some other stuff done) for £181. Hope this helps, Sam