I've posted about this before, but I've got nothing but good things to say about the SPOT Messenger.
I've been testing the device extensively over the past year and a half and I have four operating in the field at any given time with my survey crews. We were in some pretty remote locations and the SPOT gets used by each and every crew daily as part of our OHS plan. We still carry sat phones where ever we go, but the sat phones rarely get used any more. Each crew chief "scheds" in three times a day with his SPOT device, and when the unit is not sending an OK message, it's in tracking mode. I've used them on five continents in the past year or so (North America, South America, Europe, Africa, Australia) and have had no problems whatsoever.
In order for this device to be used successfully, some common sense is required. A clear or mostly clear view of the sky required, but this is the case with any device using GPS. The SPOT II has some definite advantages over the earlier SPOT I model, namely size, a user customizable message, protected HELP and SOS keys, and it automatically reverts back to tracking mode after sending an OK message (this was easy to forget on the SPOT I).
The recommended lithium batteries are quite expensive, but if you are using it daily, regular alkaline batteries work just fine (we always keep a backup set of lithiums with each unit). For the casual user or user that only plans to use this if they are in a jam, definitely stick with the recommended lithium batteries; the last thing you want is to actually need it and find out your batteries are dead.
As I mentioned in my last post on the subject, for the price, it's hard to come up with reasons not to carry one.
I was unable to find much in the way independent scientific testing of the system, although anecdotal reports are plentiful.
Here is a decent review of the system from SARSCENE magazine:
http://www.nss.gc.ca/site/ss/magazine/vol18_3/406/spotchecked_e.asp
Full pdf version of magazine here, see page 15:
http://www.nss.gc.ca/site/ss/magazine/vol18_3/Vol18-3_E.pdf
I hope this helps!