Here's a review of the Spot:
http://www.equipped.org/SPOT_ORSummer2007.htm
Also, Doug Ritter on
www.Equipped.org has TONS of information on various survival products including some PLB reviews.
SPOT is a GPS tracking device. It is similar to what is used on trucks or shipping containers by logistics companies to keep track of their assets. It uses the Globalstar satelites, which do not have worldwide coverage. Also, this is a commercial outfit, so there is no guarantee that it will be around next year or next week.
PLBs, EPIRBS and ELTS are radio beacons that use the COSPAS/SARSAT satellite system to triangulate the location of the beacon. Some PLBs will interface with your handheld gps and some have onboard GPS. I can see pros and cons of both.
I haven't pulled the trigger yet with a PLB, still in research mode, but it's definately going to be a PLB. Spot is not really designed to be a search and rescue beacon, and I'm not going to risk my life on it. Although if the tracking service was about 1/10 the cost, it might be fun to let the family keep track of my adventures, but I think you can do the same thing with some GPS enabled cell phones now.
Oh, BTW, a google search will turn up a few places where you can get a FREE SPOT if you buy the service. There's a thread on Supercub.org.