We'll be using it in January, too.
We used it in September two years ago, and even in the offseason, it was incredibly effective. We experienced a number of times where we walked onto a ride, only to discover that a line had built while we were riding. My husband, who is NOT a planner by nature and was very against being so scheduled, even insisted we subscribe again when we went the next time the next summer.
I find that if you use it in the morning, it's possible to get a TON done. Then, in the afternoon, if you want to be more flexible and relaxed, that's an option.
Recent reports seem to indicate they've got the Fastpass thing down pretty well now, which was my biggest concern.
It's only $15. For me, minimizing my waiting time is worth far more than that.