While OSX is indeed a unix, it hides this so well that unless you know what you're looking for you would never use any of the things that most people mean when they refer to unix.
I write this on a iMac I use for developing iOS applications, but honestly, if I hadn't been using linux since 2006(Ubuntu, gentoo then arch for the past 2 years) I would never have learned the use of the command line(and tools such as find, sed, grep, etc...) just from using the mac.
For me, the switch to linux was mainly because I wanted an alternative to Windows(when I first heard about Vista) and I could definitely not afford a mac(I was a student at the time). I suspect that many a tinkerer/hacker has gone down this path. In fact, most linux users have met have had similar stories, while mac users have mostly been professionals and/or design/hipster-type people. I have not met a single programmer whose main unix learning experience has come from OSX while I've met tons that have learned from linux(and a few from other unices) even if they eventually ended up using OSX in their daily work.
As I said, this is all anecdotal, but I wouldn't be surprised if these trends were more general.