I just got finished riding Frick and felt the need to spread the good word. The park is in great shape with a packed inch or so of snow covering the trails. With the forecast calling for daytime temperatures below freezing for the next week or so the conditions should stay great and most likely improve as the remaining snow gets packed down making the trails even faster. Word of caution! as the trails see continued use ice is likely to form under the snow creating some slippery conditions.
I like riding my single speed in the winter as there are no derailleurs to freeze and quit working. Also, I'm more likely to take a spill in the winter so I don't have to worry about wrecking my der. hanger. I prefer studded tires in conditions like these. They grab like velcro. The only downside is they weigh three time the weight of normal tires so going uphill takes a little patience. Good news is that when they come off in the spring you feel faster right away!