I started using the rowing machine more regularly a few year ago, and I felt it gave me very effective workout both physically and mentally. Sometimes the rowing machine feels cruelly tough and you feel "that the frontal bone grows" a little for each stroke. I love it! And sometimes hate it at the same time.
But a more complete workout in one machine is hard to find. It gives me a good all-round training in my profession as a firefighter.
A year ago I found the Concept 2 Online World Rankings where I can log in and compare my times with others and it helps me keep motivated. I usually row 10 000 meters, and my goal is to reach one million meters before this summer, and eventually to come down to a personal best time of 36 min for 10,000 meters. Maybe you will even see me participate in some of the online challenges in the future also.