...and yet, when I first got my SPOT, I'd turn it on, press OK, toss it on the dashboard of my car, and drive to work. Almost always, the message would be sent from some location ~5 miles from home, along the freeway where I had be driving a comfortable fast-lane speed. I stopped doing this experiment because the results were consistent; I can't remember one failure. I've also successfully sent messages from the SPOT in a cargo-pant pocket while walking, although not with 100% accuracy.
Over the past several months, I can think of only *one* message I sent from SPOT that I thought should have been received, but wasn't (from Union Point above Yosemite Valley: no overhead trees, good horizon, and no "no fix" alternating flashing). On the other hand, the unit has surprised me with several sucessful messages where the unit was placed on a railing of a covered porch, with at least 1/2 the sky blocked.
Perhaps there's some sample-to-sample variation--I remember this being an issue with GPS-enabled PLBs, where there was sufficient variation in the GPS chipsets that some units could not get a fix under optimal conditions.
FWIW, I don't use the optional tracking mode, which seems (based on web reports) to be much less reliable than pressing OK (or Help).