Rigor: Depends a lot on the high school you came from. For me, my high school was incredibly weak in the sciences and very strong in english, so I found USAFA english classes to be very easy, while the level at which the science classes were taught was much higher than high school (although I still didn't have too much trouble, since math/science are my strengths anyway).
Work load: Very different. In high school there were a lot of assignments that would help pad your grade going into tests, at USAFA there may be some graded homework sets, but a large portion of the grade is the GRs (graded reviews aka important tests) and finals. The homework is more to help you check your understanding and might be worth a bit, but generally not much. This basically means the work load is what you make it. If you understand the material, you can get away with doing very little (for me this was chemistry, I just understood it so I didn't have to do much, just show up and take tests). If you are struggling, then going in for extra help, asking for extra problems to work etc..is a lot of work, but will be necessary to do well.
Helpfulness: Both are very helpful. Instructors are very available and willing to help and if you go get help early, there is no reason not to do well in a class. Study groups are also common and helpful.