As Congress examines ways to overhaul the nation's healthcare system, the United Agricultural Benefit Trust has found itself in the national spotlight as a model for a proposed co-op option consumers could consider along with private insurers. The Trust works like a commercial insurer, negotiating rates with a network of doctors and hospitals, but it is owned by its members. Many of them say co-ops offer better service and are cheaper because they don't have to turn a profit. But critics say co-ops, which are not as tightly regulated as other insurance providers, are susceptible to insolvency and would not work on a large scale.