I remember hearing a while back someone from the Indian sub-continent saying "there is only one God, but He reveals Himself to different people in different ways". This could apply equally well to FTM for Mac.
I am an extensive user of FTM using the program for several hours each day. Because I include in my database a one-name study, I currently have around 105,000 people which is about 10% higher than than when the Mac version was released. I am currently using FTM2009 on Bootcamp partitions on my two Macs as I had to abandon FTM for Mac. FTM2009 (with Windows 7 64 bit) overcomes most of the problems I have experienced. FTM2010/2011 are, for me me, virtually unusable.
The first thing to say is that you do not need a version of FTM to download data from Ancestry. You need a browser and a subscription. What FTM allows you do do is to be "lazy" and merge it in by clicking.
One of the key components of the Mac version which enables it to run the .Net format is Mono. Many programmers regard the version used as having a serious memory management flaw. As your session goes along you are likely to experience the program grabbing more and more memory until it hits the 32bit memory limit. Note that for a program that can only run on 64 bit Macs, it is a 32 bit program. The extent of any problem will depend on the size of your file. My database will render a 4GB machine virtually incapable of running other than FTM and the operating system itself.
Problems I have experienced include the following:-
General speed problems. It takes some 4 minutes for the dialogue box for filtering to come up, I can't compact, I can't export a gedcom/file. Several other problems such as merging are tied up with the memory/file size.
I can't sort spouses.
There is a problem with places. It doesn't like US places which have a county which is also a place within that county such as Los Angeles.
Initially it couldn't import version 5.5.1 gedcoms. Although it will do so now, it will crash if the gedcom is not 5.5 or later.
Importing can delete the preferred parents/sposues signals.
That more or less sums up my experieces.
Others have reported problems with the conversion tool. This wouldn't affect you. You need to bear in mind that you would need to import by gedcom from Reunion or Legacy. You might need to carry out extensive work on any linked media, though not as much as moving the other way as FTM gedcoms do not include media links.
Several people have reported phantom children, thus a child might appear say five times, but only once in the index. Delete one and they all disppear.
Others have found multiple copies of media attached to facts and so on and so on.
Personally I would say that the Mac version is as flawed as the universally criticised FTM2008.
How you might find it is impossible to say. I have two suggestions if you really do want to try it and don't know someone who has it and would allow you to test drive it on their machine, wait until the new version comes out apparently at the end of the year and buy a pack that includes a bundled subscription that will cover or even exceed the cost of the program (as my copy did)
The big question for the next couple of weeks is will it run on OS Lion? I have already had an upgrade to one major branded program which otherwise wouldn't.