My mom used to have one of those fold-up kind with the hydraulics, which I tried quite a few times and I've used the concept II rowers at the gym.
There's a huge difference. Sort of like the difference between jump roping with a piece of string and using a nice jump rope with the heavy rope and comfortable wood handles. Sure, you get a workout, but it doesn't feel the same at all.
The kind with the hydraulics that I used felt flimsy and wobbly, and the handles were just a bit too long for me, so my shoulders didn't feel like they were in the right position. After a few short and frustrating workouts, I wasn't interested in using it anymore. (I believe she sold it for about fifteen bucks...)
One of my friends has a Concept II rower in his apartment, and I ask to use that thing almost every time I go visit. It feels comfortable every time, and I feel like I'm getting a decent workout. I'd get one (or something similar) for myself, if I had a good place in my house to set it up. Oh, and if the gym wasn't three miles away and stocked with several of them.
Check out rowers at epinions.com
to get a better idea of what models are best.