Going to get a little aviation geeky here.
Northwest is a decent airline. I find some of their aircraft are a little on the old side, but generally the worst that means is no seat back TV unless there has been a retrofit. The fleet age report I just ran said that their 747/757s are averaged between 14 and 20 years. The A330s are considerably newer. Generally if flying intercontinental you would be on any of those aircraft.
Northwest also was an industry leader in turbulence prediction and avoidance, but since the merger with Delta they might have changed their policies. You could actually track of flightaware.com NW flights flying considerably further out of the way to avoid weather systems, etc.
Food and cabin condition wise, my experience with NW has been on par with other airlines. Not great, not bad...
As for renting an apartment, how long are you going? If you decide against an apartment, I can recommend an awesome bed/breakfast that was about 70E a night when we went.