It is definitely worth it to have a SPOT! I bought mine for a week-long solo trek a year ago and now I never head out on solos, or group trips without it! It's great peace of mind for those who are waiting at home for us (and who often think we are crazy for doing what we do!). It's also good peace of mind for me knowing that I can get a number of different messages out where my phone often doesn't work.
I have the SPOT II and in warm weather with light gear I strap it to my upper arm and it works great. When I have a backpack I mount it face up to the top of my pack and it works great too. The times when it is less than optimal is when I'm under heavy canopies below tree line. In those periods it can be an hour or so between successful chirps.
I haven't taken it up Whitney yet, but plan to soon. I hear it works very well with a few lost chirps in the deeper canyons.
Make sure you get the track me option for $49/year. Its great fun to get home and look at where you went! It's also fun for those at home to watch your progress.
