got to be handy to check times after each practice session to see what's working especially for trying different gearing etc- lets you know how far off the mark you are- reckon looking during the session is only going to slow you down or cause a nasty misshap though - the beacons do seem to work on track days too- you just got to hide them away down in fairing so they cant see that you've got them fitted - at pembrey trackday last year got praised for not using it when they saw the empty velcro on top yoke- little did they know..
A glance down on the straight is normally all it takes. I used it quite a bit last year, more to check I was roughly on constant pace when getting tired towards the end of a race
There was time on the back straight at Snetterton to have a look and add that time to the split time you had a look at coming up the start finish straight.
The GET one I had (and probably all GPS ones) let you set splits. Mine would flash a green or Red LED as you passed the split and tell you how much up/down you were on that split (to your best lap). It was nice I keep seeing that green LED in the bottom of your vision as you worked your way through the first session.