If you would test drive a car before buying, then you will probably want to test drive a bike before buying. The same caveats about how much you really learn driving a brand new car for a short test ride apply. Yes, the car is a bigger purchase but since you are the engine on a bike it is worth comparing how you feel on different bikes. Even with similar geometries, the bar and wheel choices and contortions of chains stays and other frame members may cause the ride to feel really different or will intefact with your pedal motion in ways that you don't like.