I have a friend whose 2.5 year old son has gone their for nearly a year. This isn't a first-hand review, but I can pass on what she has shared with me. She is mostly satisfied with the program although her ideal is to get him into a Waldorf program. Waldorf is insanely expensive in the area, however, and she has a good rate at rainbow of less than $1,000/month.
Her complaints with Rainbow is that while they don't allow excessive sweets/candies to be sent to school as snacks, other junk foods are permitted. She also dislikes the conformity aspect of montessori and the way that they refer to activities as "jobs." She feels that they have their entire lives to work, why start "jobs" now? For the price, however, it sounds like a good option in the area.