Consumer Reports does surveys periodically of the performance of auto insurance companies, both with respect to policy service (applications and changes to your account) as well as claims. They rank the companies based on these surveys. USAA is usually at or near the top of these rankings. State Farm (most frequent insurer of NSX's) also does quite well.
In the next day or two, I'll try to locate that issue and provide additional info...
The most recent report on auto insurance was in the September 1999 issue of Consumer Reports. There is a lot of good information about shopping for insurance that is very worthwhile. Here are some highlights of their ratings; for more details, you should be able to find it in your local public library.
For prices, they did surveys in five cities (Atlanta, Chicago, Dallas, Los Angeles, and New York). They show the median price in each city and list the ten lowest priced companies. USAA (either United Services Automobile Association or its affiliated USAA Casualty) was the only company that was listed in the ten best for all five cities, but was not the lowest in several of those cities.
They also ranked satisfaction with the claims processing of the major companies; the higher the score, the higher the satisfaction. Scores ranged from 81 to 94. The top four rankings were:
94 United Services Automobile Association
94 Amica Mutual
92 USAA Casualty
92 Cincinnati Insurance
However, of these four, only the two USAA companies were among the lowest prices in any of the five cities noted above.
The claims satisfaction scores of the other companies insuring the most NSX's were:
88 State Farm Mutual
87 State Farm Fire and Casualty
83 Allstate
83 Farmers
85 Nationwide
Choosing a company (based on price and/or service) is only one decision in purchasing insurance. The article covers topics like how to shop for insurance, how you might get different prices from the same company, what each of the coverages mean and which ones you really need and which you may be able to skip, the effect on premiums of your driving record, family status, location, and commuting status, etc. The article is really quite comprehensive and highly recommended.