I fly them 3-4 weeks a month and I have no safety concerns. In fact they seem to have things under control from the mechanics strike, and I have not seen a rise in delays as a result.. They had replacements trained and certified lined up before the strike and it only caused a small ripple.
I gotta believe that the replacements were mechanics who had been laid off from other airlines so it was not like you got someone right out of technical school.
I would not let safety concerns stop me from flying NW.