There are many companies that offer rafting trips on the American River, and after looking at ~10, I picked H20 Adventures because 1) they were a local company, 2) I didn't want food included, and 3) price. It was straightforward to book through their website, and when I had to make some changes later on, I emailed and talked on the phone with Fianna, who was responsive and helpful in answering my questions and making the changes.
Our group of 7 adults did the half day South Fork of the American River, Lower Gorge Run. Once we were all gathered at the river campground, we were given our equipment (all in good condition) and a thorough safety talk. Most of us had no experience with white water rafting, and our guide Marina was a great instructor and paddle captain. At the beginning, she asked us if we wanted our trip to be "mild or spicy," and when the group decided on "spicy," she delivered a high adrenaline trip on the river, including spinning the raft, purposely hitting rocks, and tackling rapids head on, but each time she gave clear instructions before and during, so we were able to paddle together confidently. Everyone had a blast, and I doubt we would've had the option of a "spicy" adventure if we had gone with one of the bigger companies (e.g. OARS), which seemed to have a more rigid schedule and style for their large groups.
Highly recommend H20 Adventures if you're looking for an adventure on the water!