I've never done Rainier, but I have several friends who have. Every one made comments similar to Ken's. All went with guide services, even though a couple are experienced with snow/ice travel. Even though they were familiar with reading snow for avalanche danger, it's a different animal on a glacier. The crevasse danger is very real.
One who went with a guide service pointed out that your legs and aerobic capacity need to be in tip-top shape for the extensive, sustained uphill march. He described it as an interminable slog, just putting one foot in front of the other on steep ice most of the way. He said there was virtually no break in uphills over the two days to the summit. His particular guide service set the pace, and if you couldn't keep up, back you went. No "fast" group and "slow" group. He didn't summit because of this, although he successfully summitted Shasta the year before by one of the longer, tougher routes.
Again, second-hand information, but from sources I trust.
