I was lucky enough to ski Sunday River, for the first time, on Jan 2, 2008, after a dump of about a foot of fresh, dry powder the night before. It was one of the best days of New England skiing I've had in roughly twenty-five years. Because of the deep snow, everything was skiing a notch harder than it should (for me at least -- I'm used to New England hard-pack), so the mountain was plenty challenging. Furthermore, I had no lines all day -- every lift was ski-on (though some lifts were unfortunately closed.) There are many twisty, medium-steep intermediate trails that I simply loved, tough enough to warm up your legs but easy enough for an advanced intermediate like myself to get up some rhythm and speed.
One of the most appealing features of Sunday River is its sprawling layout. There are many different peaks and you have a real sense of exploration traveling from one region to another. The scenery is outstanding.
I had read a little about the strategy of skiing Sunday River before going -- it was good advice, and I'll pass it along. Don't plan on doing too much "horizontal" skiing -- that is, choose an area of the place and stick with it for a bunch of runs. When you're ready to move to a different area, kiss your current area goodbye because you'll waste precious time trying to back to it. Look at the map carefully and find the "most downhill" way to get to the next section, and go with that. With a little planning I was able to avoid herring-boning all day with only one exception: an unavoidable trail linking Jordan to the rest of the place called "Kansas" that is as flat as its name suggests.
Also, stick to the detachable lifts to cram in as much skiing as possible. I would've ridden detachables all day but my quads were burning so badly I actually took a few slow lifts on purpose! And it was one of the only ski days of my life where I was actually relieved that I got down to the bottom at 4:02 and not 3:59. It was a whole lotta skiing.
I highly recommend Sunday River and I will be back.