Depends on the sweater. For the most part I think they run a little big -- for example, the cashmere tees (both short-sleeve and long-sleeve) and turtlenecks. The cotton Jackie cardigan has a much more snug fit.
For me the sweaters definitely run pretty big, any of the cashmere long sleeve tees I have from them I buy in an XS and the shoulders are always really big and they look pretty boxy on my midsection. I have a few cardigans and then tend to run big also, but they don't seem to be as big as other J.Crew sweaters. I am very narrow in the shoulders so maybe that could be part of the problem.
I do think that many of this year's pullovers are very boxy in the torso. I would highly recommend calling and getting the exact measurements on the sweater in which you are interested. I usually wear a small in sweaters from Ann Taylor and Banana Republic and find that I take the same size in most sweaters from J Crew also. I called a couple of weeks ago about the marzipan cardi they are currently offering and a few other cashmere sweaters. The cashmere ones I ended up staying with my regular size, but the marzipan cardis bust measurement on a small was only 33" v. the medium's 35." Since most of their cardis in smalls are 34," I ended up sizing up. I am glad I did as I usually wear a 2 or 4 in J Crew's camis and I would not have been able to wear a small in that particular piece as it would have been a tighter than I like in the sleeves and bust.
Their sweaters are True to size depending on if you want a snug fit or not. I have gotten sweaters XS to M depending on what look I am going for. If you want to fit more layers underneath I would go for the size above. I think their shirts/tees/knit shirts run "bigger" but otherwise, I believe that most of their items are TTS.