I use a Macbook Pro for FFXI, but not as my main gaming system. It works great, but gets pretty hot for obvious reasons.
FFXI will work in parallels and I use parallels (with Vista installed) to run FFXI while doing less involved tasks in FFXI (i.e. crafting, farming, checking plants, checking AH, etc.) I have used it for events like Dynamis and such but I normally have to turn the graphics way down due to slowness.
For events, boot into Vista using Boot Camp so everything can run the way it was designed to run and not through some emulator. FFXI on my machine while running Vista natively works just as well as running on any other laptop.
On my machine I can bump all the graphics up and resolution up as high as I want and experience little to no slowness with the game.
Hope that helps.
ADD: I've had zero stability problems. I made my Vista partition with boot camp and pointed my parallels software to run this partition while in OS X and I've had zero issues doing so.
The nice thing about running FFXI in parallels is that you're able to still browse the web via OS X while play FFXI Windowed via Parallels. Which is great since you won't have to worry about visiting malicious websites and such as much.
On that note, I found FFXI seems to operate better (faster) if you run it in full screen mode and let parallels do the "Windowering" instead of SE themselves; however, I do not know how FFXI would operate using the 3rd party Windower that everyone out there seems to use ... I don't use that.