I have experience with both the 1st and 2nd generation (see link for writeup). Both work well, but the 2nd generation (Spot II) is much better. I have compared the 1st and 2nd side by side over a 14 mile hike - SPOT II is the clear winner.

I like SPOT - there's really nothing like it. Just tonight I was checking in on my son's Elk hunting trip - he has his SPOT II in track mode.