The Spot has been discussed on this and some of the other forums since last August. It is a private service and should not be confused with the 406 Mhz EPRIB locator beacon, which sends a message to international SAR/SAT, a multi government entity.
Spot can send a pre-programed message to a designated person, or a distress message, or you can pay extra for a "Tracking service" which allows people at home to follow your trail, when hiking or boating.
The major criticism is that the GPS is not as sensitative in picking up a location as many hand held GPS units. The unit relies on Qual Com satellite data network (private) which has limited world coverage (vs full world coverage for the 406 EPRIB). Even though Qual Com's phone network does not seem to be good, the data network seems to be much better. The "distress" message goes to a private service, who then calls what they feel is the nearest "911" operator. Then that contact has to launch a search--not much follow up, nor real guarantee. Vs the 406 EPRIB which goes to government agencies who have a protocol, contract the relitatives, check for a float or travel plan, then notify the nearest government agency to start a search. I feel that your chances of being found are much better with the 406 EPRiB.
The cost for SPOT is less, ($150), but there is a yearly charge of $100 to keep the unit activated, plus a $50 charge if you want to use the tracking feature. We purchased a GPS enabled 406 personal locator beacon for $500--or $150 less than the spot over a 5 year peroid. The 406 is tried, you have a government agency looking for you, world wide coverage, and a much better chance of being found.
I do believe that there is a place for this--if you want the folks back home to track your progress, and if you can be clear what protocol will be used for search if you activate the "911". But lets say you are at Lake Powell, and activate "spot"--who is notified? it is by lat and long, so do they call the nearest sheriff in Utah, or Arizona, or the operator in Page AZ. The 406 would activate a search, involving CAP, Airforce, Park Service and local agencies.
More to read about "Spot" on Panbo:
http://www.panbo.com/archives/cat_safety.html