I love the simulator. I use it to practice manouvers before attempting it on my real planes. Some, not all, downsides to the sim as I see it are:

1) Wind in the sim is not realistic. Real wind will have a buffeting effect on your real plane creating pitch, yaw and roll variences that you must learn to counteract. In the sim it just pushes you plane some depending how strong you set it in the sim.

2) It seems that in the sim the planes relitive speed is slower. I can fly a jet plane at 200 mph in the sim and land it, I seriously doubt I could in reality.

3) Without practical aplication, real training from a real person, you will develop habits that work in the simulator that do not work in reality. ie. Most planes in the simulator will allow you to simply chop the throttle to an idle and glide in for a landing. Try that with a warbird, and some sport planes, and you will be bringing home plane parts instead of a plane.

4) The plane in the sim is perfectly tuned and trimed for flight. If you expect that in reality, you will be soarly disapointed.

These are just some downsides. I'm sure there are more. I am in no way saying do not get a sim. Contrarilly, I say get one. Just try to get an instructor too.