I don't have one, but would like to get one. Concept II seems to the be one everyone recommnends. It's pricey, but well worth it, from what I've heard.
It looks like we'll be getting a Concept 2 for Christmas (or the New Year)! I had seen advertisements for rowing machines that use water flywheels, but they are very expensive. I wasn't sure if they were worth the extra money. It seems that the Concept 2 is more moderately priced in comparision.