To compare the safety and tolerance of oral ganciclovir at a double dose 3 times/day or a single dose 6 times/day to IV ganciclovir given for 20 weeks of maintenance therapy. To compare the time to progression of cytomegalovirus (CMV) retinitis between two regimens of oral ganciclovir and IV ganciclovir therapy given for 20 weeks of maintenance therapy. To describe the efficacy and safety of double dose versus single dose oral ganciclovir in patients who have a progression of retinitis while on the originally assigned maintenance treatment. To describe the safety, tolerance, and time to progression of retinitis during the 52 weeks of oral ganciclovir maintenance therapy in people with AIDS. To describe the safety and tolerance of oral ganciclovir maintenance therapy when given concurrently with antiretroviral treatment (e.g., zidovudine, ddI, or ddC). To describe survival of people with AIDS and CMV retinitis.