Speaking from my own limited experience (about 10 trips with them), they are a very satisfactory carrier. Very good on-time performance, no problems with connections, and all-jet service.

They are far superior to Southwest, with pre-assigned seating and a business-class cabin.

Sometimes the lines at the check-in get a little long, but have you ever checked in at Northwest in Detroit, or Delta in Newark? They're really not out of line here.