Getting back to the original questions:

I've used both Costello (for my Russia AC-4) and Avemco (for my
DG-300). I'm a 100-hour glider pilot with an SEL rating (about 200
hours TT).

Costello was decent for my Russia, but they require full payment in
advance and came across as very limited in terms of options or
flexibility. Costello also now balks at "winter storage" or "non-
flying status". I tried to work that out with them when I bought my
DG-300, and was told that I could not switch over to this kind of
coverage for any shorter than a 1-year term. :-/

Avemco has been a bit more pleasant to work with, IMHO. They were
more flexible (multiple payment and coverage options), and provided a
slightly lower quote than Costello for my DG-300. They provided a
pleasant surprise, too: there was a SNAFU with my paperwork (USPS),
and when it finally went through I'd racked up a few more hours of
glider-time - pushing me across the "150-hours TT" threshold. They
called to confirm everything, and when they found out about my added
time, they applied it to my account and lowered my premium. It wasn't
a big change, but it was a nice surprise!

There is a caveat with Avemco, though: They are primarily focused on
powered airplanes, and their policy is not to insure any aircraft that
they don't have on their master list - so they can't provide coverage
for some of the lesser-known models of glider (like my AC-4). However
they do seem savvy about the major brands and models, and they seemed
to understand the insurance needs of a glider pilot (trailer, towing,
etc) just fine.

I have never had to file a claim with either company so I can't
comment on that side of things; but I know of a Discus owner and
another DG owner who also have Avemco and are pleased with their
coverage and service.

I think the bottom-line is that you should always shop around and then
decide for yourself which company provides the best _combination_ of
coverage, service, and price.

Good luck,

--Noel