I flew with NW domestic in the U.S. six years ago and they were HORRIBLE. Flew from LAX to MSP with a DC-10 and I was stuck in the middle seat of the middle section and beside me sat this annoying little kid who just wouldn't shut up
Then I flew MSP-YEG and back with their DC-9's which were ok (although I nearly froze my rear-end off on them) and then MSP-MCO on their A320- takeoff was delayed by about three hours, and their previous flights weren't very punctual either. To cap off I then flew MCO-BMI (Baltimore) with a stoppover in Detroit, also with the A320, which was somehow manageable.
In short, unless they have improved significantly in the last few years, stay away from Northwest!