I've had my SPOT 2 for a year now and got it mainly for an 8200 mile, 4 week bike trip last summer. I got a ram ball headded bolt replacement for my handlebars and got a RAM cradle. It tracked flawlessly, batteries lasted about two weeks of every day tracking and an OK message once or twice a day. I left the spot 2 out on the bike in downpours, riding in downpours, and such and it's still working so no issues on the waterproofness.
Is it expensive yeah, but it was nice for my dad to login to a website and see where I am. Also when I was getting close to his place he would just check and knew right when I was going to get there which was nice. The tracking isn't granular enough for plotting out a trail though. Mine sent a track location every 15 minutes or so. Moving at 65-70 mph makes track acquisitions less frequent.
It took about 90 minutes but I did get an OK signal to send from a campsite in the bottom of a ~100 ft somewhat-narrow canyon. OK signals took about 15-20 minutes to send in an open location.
Here's the mount location on my bike:
Overall I like it. I won't keep the tracking next year mainly because I'm going back to school and money is a bit tighter now. I also have a handheld GPS unit for making tracks.
If money was no object I'd probably supplement the SPOT with a PLB and keep that on my person so I have something on the bike and in person. But money is, so the SPOT alone is acceptable enough for me.