I was a rower in college... I spent many hours of my life on these machines in the winter and during hot summer months. Concept2 is the way to go. Some of the cheaper ones have nicer computers, etc....but the key is to have solid resistance, a good slide, and solid chain mechanics. The Concept2 has all 3.
Just remember to study the fundamentals of rowing before jumping on the machine. If you just pull the handles, your form could be really poor. With proper form, you can maximize your workout and get more accurate numbers from the computer. If you're new to rowing, you should shoot for 2 minute 500 meter splits. When I was in training, I was usually pushing 1:30-1:40 splits, but couldn't keep up that pace now...I'm too lazy!