I've carried a PLB ever since they were made legit in the USA in 2005 in the High Sierra. The latest PLB by McMurdo Fastfind is 5.3 oz. Well worth the weight. Last year on the JMT, as reported on the JMT Yahoo Group, a guy had his life saved due to having the Spot handy. He needed an appendectomy surgery the same day he had a severe attack on the trail. Docs told him he might not have lived if he had not been operated on the same day. I talked to a Ranger familiar with that situation.
This year, we'll be using a Spot, as I sold my older PLB which weighed 13 oz. Another member of the group has a Spot.