It is beautiful up there. The ride around the rim is spectacular, but there are a few hills. In fact, the elevation gain riding around the rim exceeds the elevation gain from the 20 mile climb up to the rim! My memory from last year is a bit fuzzy, because I was trying to keep up with someone faster than me, but I seem to remember at least a couple of those rollers around the rim were a couple of miles long, and steep enough to get your attention. Not a brutal climb like Sierra, but steep enough. You'll need to carry enough food and water (it's not that long around the rim, so that shouldn't be a problem) - the century is a supported ride, so I wasn't looking for any place to refill, but I don't remember any.
The nice thing, though is that it is tough to get lost. Just keep the lake on your right (or left if you go counterclockwise) and it's pretty hard to get lost :-)