Class III straight up at the current flows, but is still a fun and scenic stretch. There are 4-5 rapids that stick out in my mind on this stretch, all straight forward wave train type rapids. If you have solid class III or IV skills you will probably kill it, if you are just getting started, this is a good stretch to cut your teeth on. It is a long run from Sims to the reservoir (not sure where mosquito is) so get an early start. The one thing to always stay heads up for on the Sac is river wide logs and strainers, they seem to fall from the hill side on a somewhat regular basis. Haven't seen any wood warnings on boof lately however so you are probably good, just heads up.
good write up above.