No lines; huge; not horrifically expensive; something for everyone
Can get windy; was a little icy when I was there
This was my first time skiing outside of California, and I have to say I'm impressed. As a regular Mammoth Mountain visitor, my standards are rather high, but it didn't seem to matter here.
First off, it's significantly larger than Mammoth. In fact, if you look at skiiable acres (not including backcountry stuff) it's half again as large. Not only does this mean there's tons of skiing, but it also makes the lines shorter because everyone is spread across more terrain.
Second, lines were practically non-existent aside from the two express lifts at the lodge, which are used to get to some of the easier runs and the rest of the mountain. Even those took a couple minutes at the most to get through - a far cry from the 15+ minutes you'll have to wait on occasion at Mammoth.
Despite being there in what is arguably one of Big Sky's worst winters in recent history, the snow wasn't TOO bad. It was a bit icy in places, which made skiing a bit difficult (not helped by the cheap rental skis with nothing for edges), but after talking with some of the locals, I came to the conclusion that the snow is actually pretty good most of the time.
It does happen to be a bit windy, especially in the afternoon, but wear a good jacket, gloves, and maybe even a ski mask, and you should be fine.
(Also, if you tour Montana State University, ask for coupons and you'll get some huge discounts on lift tickets)