I have one and use it. Like it very much. Got it originally, and have used it mostly, for hiking and hunting in the Colorado mountains. Now that I live in Tampa I will use it when sailing. You just have to be aware of its limitations.
For one thing, it does need a clear view of the sky. I suspect that even using it from under a bimini would be an issue. It can also sometimes take several minutes to successfully send a message, especially if you simply turn it on and hit the OK button without giving it time to "find" itself. Finally, since the older units provide no positive indication that a message has been sent, you have to keep an eye on it to see the indication while the message is BEING sent. Otherwise you just press the button and hope.
But with these limitations it is very useful. When I went hunting by myself my wife was very comforted by the nightly "I'm OK" message. When she went to Spain to visit our daughter it was neat to get an e-mail and see where they were in Madrid, Salamanca, Barcelona.