I'm a great fan of Backpackinglight.com, the Web site, and magazine, dedicated to shaving the kilos and grams off your back, your body and your feet. Much of BPL's advice is relevant to sea kayaking, particularly for those long distance trips we yearn for.
BPL has recently posted the most comprehensive review of the SPOT satellite messenger yet undertaken, posing the question, does this highly anticipated technological breakthrough live up to the hype? Interestingly, BPL's reviewers are unable to rate SPOT, as they believe SPOT "has yet to deliver its promised functionality and message delivery reliably."
If you're seriously considering buying a SPOT, I thoroughly recommend subscribing to BPL - well worth the money - and reading this review. But I would also suggest that you read the review keeping in mind that the majority of tests undertaken were by backpackers out in the wilderness, where a clear and unobstructed view of the sky is not often possible.
With my own eight or nine months of testing SPOT (and a review I wrote for Sea Kayaker magazine is out in the next issue), I have found SPOT to be very reliable, when used for kayaking, where an open sky is usual. I'm still a SPOT fan, and so is FliesWithKiwiBird.