AIM bills itself as marketing organization, but I think that's a fancy term for saying they're an insurance broker. I'm pretty sure they specialize in coverage for parent groups, school districts and municipalities. While you might not have heard of Discover RE (short for reinsurance), the actual company underwriting the policy, it is part of the St. Paul group of insurance companies. St. Paul has a great reputation, and an "A-" financial rating from AM Best, a company that rates the strength of insurance companies.
The only other large broker of which I am aware is RV Nuccio and Associates, and they sell policies through Fireman's Fund (which also has a great reputation, and an "A" financial rating--not significantly better than St. Paul). If you're an NPN member, you probably won't get a better price from Nuccio than what AIM is quoting you.
That said, the relationship your PTO/PTA has with the broker is probably the more important factor, because you'll easily deal with them more often. You might want to call and ask some specific questions regarding claim service, like who they have available to help you, what kind of assistance they provide you in filing the claim, what kind of turnarounds they have experienced, etc., and just see how comfortable you are with the answers.
I'm under the impression that AIM and Nuccio pretty much have a lock on the market, and you probably can't go wrong with either for the price of the policies they can provide, but I don't have first hand claim experience with either.
