I am a fan of rowing, I do it every Monday as part of my XT, on a Concept 2 rower (air resistence / fan type).
It is totally devoid of any impact but moves the legs through a large range of motion and helps strengthen the upper body, the all important core, plus areas of the glutes I was previously unaware of.
I would advise starting with just a few minutes with a medium resistance and building up a minute at a time, I also alter my grip every minute from the conventional overhand grip to underhand (as you would hold a barbell for bicep curls) to target different areas of the upper body.
Finally, if doing this at the gym, try and resist racing the great oaf who will inevitable get on the one next to you, put the resistance on max and mimic James Cracknell for 4mins before wheezing, spluttering and getting off, at this point I find a nonchalant smile works best.
Hope this helps.