I have a Concept 2 at home which I use about 3 times a week, in the Winter that gets upped to about 5 times a week.
Technique is crucial and the Concept2 UK forum is a very good place to get solid info. I used to be a gym warrior cranking the lever to 10 and going at a spm of 35 and then wondering why I could never get a 10k under 40 minutes.
Rowing is very conducive to bike fitness as the power from the stroke comes from the legs not the arms.
For a good training program I would recommend the pete plan over at c2