The big advantage of a rower in is the variation of workouts you can do - it's not just about rowing at break-neck speeds. By adjusting the resistance you can alternate between aerobic and anaerobic training and vary your sessions from a gentle exercise to an extremely demanding session.
One hour of strenuous rowing burns around 800 calories!
Of course it will take time to build up to this sort of session if you're currently unfit. But compare the calories burnt on a rowing machine in one hour to walking (300), swimming (600) and even running (650 – 700). And you can do this in your own house and don't have to get wet!
However, we recommend you learn how to use your rower safely as soon as you start a new fitness regime - see rowing technique. One big advantage of rowing is that if your technique is good you can really work your body hard with vigorous sessions without risk of injury.
And note it's always advisable to see your doctor before you begin a new routine.