I think they're pretty good, Cosmo...
A co-worker of mine has one of those for the same reason: limited space. She likes it, and uses it a lot.
In my yoga class, we were taught an exercise that simulates rowing, without having to have the equipment; it worked great for my arms and for strengthening my upper back.
You may want to check out Consumer Reports and see if they have anything to say about particular models of the rowing machine.
🙂