Responding to Gerry Brucia's question about reliability of sending SPOT OK messages.
First, as Ryley said, your agreed upon protocol with your home-base should be that an absence of messages can not be interpreted as a problem. "...protocol is this: If there are no tracks or SPOT2 messages, it does NOT mean that we have a problem. It could be one of many reasons, but you should NOT worry about it. (dead battery, device lost or damaged, etc). In this case revert to protocols used before we started carrying SPOT2: assume all is well until 24 hours after our expected trip completion time, at which point Responsible Party should notify the appropriate agency to initiate SAR."
A PLB is undoubtedly a more reliable device, and if you require that level of service, you might consider carrying both. Unfortunately, a PLB doesn't allow you to proactively tell your wife that things are AOK, which is what you were trying to do. A Sat phone allows the person in the field to know that the recipient got the message, which SPOT does not. However a Sat phone is not necessarily more reliable than a SPOT - the difference is that you know if the recipient got your message.
Second, you don't need to stand in one place for 20 minutes after initiating the OK, you can just leave the device on and continue walking or cycling or whatever you're doing. The device doesn't need to be still to work. To increase the likelihood of a message going through, you really should plan to leave the device on so that it transmits multiple times.
Third, I have had >90% transmission success for both Tracking and OK messages when using Lithium batteries, even when they are mostly drained and the battery light is red. On the other hand, partially drained Alkaline batteries give me a success of something between 10% and 50%. In other words, do everything you can to use Lithium batteries.
Fourth, I have now had two SPOT-2 devices COMPLETELY FAIL while on hiking trips, one in May2011 and the other in Oct2011. The GPS chip stopped working, and the devices were dead-weight. In both cases SPOT replaced the unit. However, this level of reliability is a huge problem. This is one reason I think it's critical that the home-base knows that an absence of messages does not necessarily mean there is a problem. I'm still a fan of the SPOT concept and functional design. However, the reliability problems I've had make me eager to see some other company offer a competitive product.