I have used a SPOT for the past several years.  I go solo for 1-2 weeks at a time in BW or Q and I do that 3-5 times a season.  I send an okay message every day.  When I check the map on my computer after I return I find the locator has set my position exactly. 
I did freak out my "pickup" person the first time I hit the "help" button....repeating urgent messages on his cell phone....until he realized they were coming from the landing where I was waiting for him, a day earlier than planned.
I have not had any issues with accidentally sent messages or with messages that did not get sent.  On occasion I will have to move the SPOT around camp to be sure a message gets sent; mine has a message sent function which lets me know if it has worked or not.   :thumbup