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.
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