I can't answer on the Mac question, but I have the PE770. I don't use it much so it's pretty well suited for my needs. My only troubles with it so far have been user error.
The only gripe I have about it is that the LCD screen is very small and has poor resolution, so it can be REALLY hard to tell which step it's currently stitching out. You sort of have to keep track of that on your own.
The designs I have were all purchased online and downloaded directly from the internet - all of that is "neutral" as far as what type of computer you have. I don't expect you'd have any troubles doing the same with a Mac - just make sure you're buying the proper format for your machine (PES). Buying embroidery software, however, is a different story and will be computer-specific.