I was a bit worried about this as well, but it has been fine for my Mac-based business. The iPhone app is basic, but extremely helpful as far as what it does.

There are some Mac-specific answers in the knowledge-base and the forums, but really I mostly interact with RingCentral via a browser (not a problem on Safari or Firefox), the iPhone app or the VOIP handset itself.

If you have specific questions, I would be happy to see if I can do whatever you're concerned you'll not be able to do and let you know.

edit: I don't really use the SoftPhone or fax from within applications. Some of the finer details of this stuff may end up being PC-only.