Mine was so unreliable as to being useless. My wife would panic and repeatedly call my cell because she would check on me online, and it showed I hadn't moved in hours when she knew I should be rolling. It was her that said " that thing is worse than useless, it actually induces anxiety I don't need", so I cancelled mine.
If it worked properly/reliably it would be a great idea. However I, and the Spot company could just not get it to work properly/reliably.