Sandi Bergevin has a terrific internist, and she adores the ophthalmologist who has treated her glaucoma for the past 15 years. "It takes a long time to build up the trust," she says. "I feel he's taking care of me and is not going to let anything happen to me." But the 55-year-old hospital worker is not so sure about her HMO. Because its vision plan stipulates only three office visits a year, getting the care she needs usually involves a long series of appeals. And she worries constantly that things could get worse. Not long ago she learned that her employer is planning to switch to a different health plan--one that may not even do business with the doctors she knows and ...