Hey Gwynedd,
Here is a quick link to a thread on this started yesterday.
http://www.sim-outhouse.com/sohforums/showthread.php?t=3697
I have it and I am looking at getting an Apple as well. X-Plane was originally Apple based.
X-Plane isnt as polished and refined as FS9 and FSX is, but its a huge and very nice platform none the less. You can change the keyboard controls to be the same as FS so that you can start flying in it fairly quickly, but some controls are very different from FS. (You'll read my input on it in that other thread).
There is a huge following of X-Plane people at their website in their own forums too that have lots of tips and all for flying in X-Plane.
How do you like your new Mac? I just recently picked up the new iPod Touch and cant beleive how cool the OS is in it. I will be getting an Apple for sure now.
Bill