I fly to Chicago (ORD & MDW) every couple weeks. There seems to be plenty of capacity; in fact they recently reduced (now restored) a daily flight to/from MDW. The fares are very competitive -- $125 the last time I looked, we're still flying a bunch of $59 tickets I purchased a while back.

If the Chicago area is your destination, also take a loot at MKE & especially MDW and maybe RFD.

UA & AA also flight there direct, DL connects to ORD thru CVG/ATL/SLC & to MDW thru ATL.

DTW is often tighter but NW has routed us through ORD & MDW number of times when we were travelling between MSP & DTW.