Being able to easily store your rower is a huge plus. Make sure you investigate how well the machine stores, if it folds up or comes apart easily to be able to put it in your closet, or in other storage areas. As an example, the Concept2 Model D rowing machine folds up nicely by a simple pull pin located in the middle of the rower. Wheels on the bottom allow for easy moving.
I kayak (ocean and flat water, not river), so I was drawn to the WaterRower over a flywheel type rower (which I have used in gyms) because I wanted to hear the sound of the water, which I miss hearing against the hull of the boat during the winter when I can't get out on the lake. I also like the way the paddles engage in the water; it's a very smooth pull and release, and moving the mass of the water instead of a dialed down tension feels more natural. As someone else also commented, the seat (at least on the wood version) is incredibly smooth and solid; no wobbling at all. I generally listen to music on my Nordic Track ski machine, but with the WaterRower I am content to listen to the sound of the water; working out on this is somehow both energizing and calmingly meditative at the same time. In the event that you want to use it while watching tv (this is one use to which I wanted to put it), it is quiet enough to hear the tv without having to turn it up.
Water rowers are just a recent invention followed by the air rower, which utilizes paddles and water in order to create the resistance. While there are a lot of people who prefer the air rower compared to this one, there are also people who prefer the water rower because it has a smoother, quieter feel of the action, especially when emulating the movement of water rowing. Like the air rower, this machine is able to burn calories and work up your muscles through offering resistance to your movement while sitting on the sliding seat. The more resistance there is, the more it makes it harder to work on the machine and your workout becomes more challenging.
Rowing has long been recognized as the perfect aerobic pursuit, with naturally smooth and flowing movements that don't tax the joints but do boost the heart rate. Now you can take your rowing experience to the next level with the WaterRower Natural rowing machine. Using the same principles that govern the dynamics of a boat in water, the WaterRower Natural is outfitted with a "water flywheel" that consists of two paddles in an enclosed tank of water that provide smooth, quiet resistance, just like the paddles in an actual body of water. As a result, the machine has no moving parts that can wear out over time (even the recoil belt and pulleys don't require lubricating or maintaining). More significantly, the water tank and flywheel create a self-regulating resistance system that eliminates the need for a motor. As with real rowing, when you paddle faster, the increased drag provides more resistance. When you paddle slower, the resistance is less intense. The only limit to how fast you can row is your strength and your ability to overcome drag. And unlike conventional rowing machines, which tend to be jerky and jarring, the WaterRower Natural is remarkably smooth and fluid.