Rowing is an excellent form of exercise for several reasons. It is a user defined exercise, which means that the user can determine all of the movements, from angle of knee bend to how far to pull the rower. It is one of the few aerobic exercises that actually engages the upper body (biceps, shoulders and core). Additionally, the seat height on a number of rowers is the same height as an office chair (about 19 inches) meaning it is easy on and easy off.
Resistance is provided in one of several ways; wind, magnetic, wind/magnetic combination and water. Wind resistance alone, as in the Concept II, will only yield a resistance factor of about 3 on the resistance scale of a wind/magnetic or water resistance rower, which could have levels that go as high as 8 (Bodycraft R400) or 20 (First Degree E-520).
Consistent resistance is a major issue with some people. Because most rowers utilize a flywheel, there is a need to catch up to the moving flywheel after the first movement. This results in a resistance lag for the first 6 to 12 inches of the rowing movement. In water resistance rowers, where the tank is vertical, this is not an issue. Since the water sinks to the bottom of the tank and the impeller blades are in constant contact with the water, there is consistent resistance throughout the entire range of motion. Before you purchase a rower, make sure you try as many as possible so that you get the one that fits your needs.