Guess it depends on what you're looking for. To say X-Plane is not "as good" as FSX is a subjective statement of personal opinion. I read that FSX has better scenery, but X-Plane is more realistic from a control/procedure standpoint.
Some people also say that FSX uses flight characteristic charts to model flight ability for planes while X-Plane uses an airfoil physics model to determine flight ability.
I jumped on X-Plane, because I wanted the experience of flying a plane with all the details. I had FSX and thought the tutorial was great, but I don't think the difference in scenery makes me feel like I'm looking out of the window of a real plane any more so than X-Plane.
Blighty Andy asked if there were any flight simulators for Mac. There is and it's also developed on a Mac, despite being cross-platform. I purchased it and am enjoying it, but I'm not going to sell it as if I'm being paid a commission. Besides, I rather enjoy supporting Mac software wherever I can so that developers will continue to make products for a Mac and I can spend less time running Windows on a Macbook.