We have been using a SPOT Messenger for a bit over 1-1/2 years. We originally purchased the device for our daughter to keep track of her travels while she and some other teens were sailing a Beneteau 50 from St. Martin to Trinadad during the summer of 2008. We then loaned the device to a friend to keep track of his travel from Nevis to Bradenton Florida in November of '08 for which it worked very well--well enough that I could time my drive up to the marina to meet him when he arrived in the middle of the night.
Since then we've been using the device to keep track of our own travels. Lately we implemented a SPOT Adventures page that keeps track of all of our travels (when the tracking mode is activated). For one thing, our yacht club has a rule about active verses inactive boats and there has been a bit of debate about whether one's log is sufficienbt evidence that one has an "active" boat and is not just using the marina for in the water storage. Our Spot Adventures page is pretty irrefutable evidence (so long as we remember to turn the thing on). Our SPOT lives in a mesh bag suspended from the stern pulpit and seem to work without difficulty. A representitive SPOT Adventures page is shown here: Daysail to Egmont Key - Sailing - Spot
We have purchased the full service package with Tracking and GEOS SAR service (which must be purchased separately). The device has proven very reliable although I do not consider it a replacement for and EPIRB.