Mid Jan is a great time to experience the the best skiing in the US. Usually this time of year we have lots of snow, and just about the entire mountain open (most everything on the map). 00'-01, and 99'-00' were both crappy seasons, and we still had about 90% of the terrain open. The biggest factor on how much of the mtn is open is wind, and weather. The upper mtn closes the minute that clouds sock in (this is a good thing), and frequent strong winds can also close the upper lifts, although the new gondy runs in some scary winds. During heavy snowfall, the mid mtn areas of chairs 3 and 5 often close due to avy danger from above. Head straight for chair 22's steep protected chutes for epic pow under these conditions. Of course, you have a great chance of getting good weather, and skiing the top of Mammoth. This area has all sorts of sweet steep terrrain. Cornice Bowl is not what it used to be with nightly grooming, but the rest of ths summit ridge is usually left for the wind to groom, and what a great job it does. Some of the best snow is usually deposited on Dave's, da Noids, and the Wipeout Chutes.
This is one of the slowest times of the season here, the weekends will be busy, but nothing like the Holiday periods. A weekend like MLK can see 22,000 people on the hill, but Mon -Thur expect more like 5000. An average Sat is usually more like 12-15,000. Even on the busiest days, some chairs (21, 20, 25) may have no line whatsoever. Main Lodge has 6 high speed lifts, while Canyon only has two, so on the busy days ski out of Main. Chair 4 of of Minaret Rd (on the way to Main) is a great busy day base, but has no SS, food, or rentals.
If you have any specific questions, drop a PM.