Some friends and I spent a day at the Northstar downhill circuit and rode everything from blues to double-diamonds. There was a good variety of trails, from fast and flowy moderates to some jump-fests diamonds to some technical butt-clenchers. The trails were generally very well maintained, and while a few key runs were closed due to storm damage, we had no trouble lapping some favorites to keep ourselves amused. And many trails were manageable by a range of skill levels -- if it's not hard enough, you're not going fast enough.
A place like Mammoth has more scale and a bit more gonzo features, but we were very happy with out experience at Northstar. Our only qualm would be the price of rental bikes -- it's painfully steep, more so than anyplace else I've looked. I brought my bike so didn't have to worry about rentals, but my friends opted to go hiking the second day rather than foot another day's rental bill.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.