A bike that just folds in half may not be much of a space-saver. I bought an old folder but have since found that it's more convenient just to leave it unfolded; it's small enough to carry, can be parked behind my desk at work and in my storage locker at home (also a small urban condo). I've taken it on public transit unfolded during off-peak hours. I think I'll be folding it when I lock it up outside, which I haven't done yet, but that might be the only time.
Don't buy anything that you haven't had a chance to try out in person, because that's the best way to find out how easy they are to fold and carry, and how little space they take up.