I have a Garmin Edge 510 and it is a piece of crap that hasn't worked since day one. It shuts off mid ride (sometimes in less than a mile) without warning much too frequently and then is difficult to start back up.

Every time I contact Garmin they tell me to reformat it, download this, reset etc. Everything but taking responsibility for the defect and making it right.

I'd stay away from Garmin if I were you.