I've run CIM 8 times with quite a few of those in the low 2:30's range. I agree with most of what's been written above. It is a rolling course for the first half, and then it flattens out pretty nicely except for the dip in the 22nd mile (guessing here) just after you've crossed the river. I think the big factor with CIM is the weather. If you can enjoy a non stormy day, you will have a course that can run quite fast. If it's stormy, it's a very difficult layout. The course (point to point) runs mostly Southwest from Folsom to Sacramento and when storms whip through the valley up here, the winds always come from the South. This means that you have to deal with headwinds And rain during bad years. Of course the weather seems to be bad every 4-5 years or so from my experience. Unfortunately 3-4 of my 8 attempts were on really bad storm days.
As with most marathons even splits would be recommended. I believe in my PR year at CIM i ran the two 1/2's within 15 seconds of one another.
The crowds are fairly slim, but the volunteers at the water stations are always great. Depending on the pace that you're running it can be lonely out there as well, with open spaces between runners and packs, especially for those running between 2:15-2:40ish.
With all of this said, i enjoyed my years running CIM and did run my lifetime goal on this course.