This is a VERY important subject.

I use SPOT II. As DF4 says, Works as advertised.
There has been discussion after discussion as to which PLB is best. Some have more options but cost more. Some have fewer options but cost less. Some work on one system, some on another. The PLB discussions often follow the "which is the best concealed weapon" discussions. After all is said and done the answer always seems to be reduced to "use the one you have, can afford and feel comfortable with".

So here is my best recommendation: Study them, yes. But most important, Get One, carry it with you all the time, turn it on and use it. They all work and any of them is so much better than none of them. Any one of them could save your life and more importantly they will provide so much relief to your loved ones left behind if you do crash and die in some remote location. Please, just get one and always use it.