ELT's are not good, and not the solution for Air Transport
I've been involved in tracking down ELT's for 30 years. In my opinion, they are mostly junk. All but one that I found was a false alarm - usually triggered by a mechanic, or by being thrown in a drawer at home by a careless aircraft owner (the latter leads to some interesting 3AM visits, BTW). In the past week here in Arizona, there have been 5 false ELT alerts that I am aware of, and every one was in an aircraft junk yard. There have been saves due to ELT's, and the ELT's are more likely to work in an event where there are survivors.
In many SAR events for missing aircraft, we find the aircraft but the ELT did not go off or was destroyed. Often the coax from the antenna to the ELT is cut.
In the vast majority of these events, this doesn't result in a failed rescue, as the passengers are deceased due to the same forces that destroyed the ELT or coax. But, it does result in hundreds to thousands of man hours and aircraft hours, many of the hazardous due to mountainous terrain. As we have seen with MH-370, a missing aircraft can cost a lot of money and time, and puts searchers at risk, not to mention the heartbreak for the families.
ELT's could be better, and suggestions here have addressed a number of the issues. For air transport, though, I think the best solution is near-real-time tracking. That avoids the ELT issue and solves the problem. It should not be too expensive, these days - the technology is just not that hard (I speak as an engineer).