well, I can tell you that when I first got my bike back in 2002, USAA was insuring bikes. however, they quoted me an insane rate of over $600 per year.
went with progressive through hd and got the same coverage for $125 per year. I've been with hd/progressive ever since. no one has been able to beat their rate year after year, even with their occasional rate increases. I've been able to offset those with increasing my deductible. with the disappearing deductible, keeps it at 0, even though I pay for $500.
except this year. with hitting a deer before Christmas, come renewal (june) i'm either gonna have to hang with a high deductible, or pay a little extra. i'm leaning towards paying a little extra. the deerstrike did over $3,000 in damage. the good news is that progressive handled it very quickly, professionally and with a minimum of hassle.