I am the proud owner of a used iMac. I just haven’t turned it on yet. In fact, it is in a box half way here right now.
I am starting an iOS project but I also hope to port the Linkage program to the Mac at some point. Although the Mac itself is not a very widespread computer compared to the Windows PC, having some experience with it should look good on my C.V. (or resume for old-schoolers). The iOS experience will be of a much bigger benefit since app development will only grow in the future. At least for a while until the app market is saturated.
I think that custom apps for business use might grow somewhat in the future. It seems like there are lots of ways for businesses to move away from paper on use tablets instead. In fact, my iOS project is for a business that deals with mobile workforces and mobile form handling.
I also wrote a web app that mimics the iOS look and feel (http://blog.rectorsquid.com/programming-projects/auto-responder/) and it might be cool to do a real port of it to iOS. But then having it on the web and accessible from non-iOS devices is quite handy too!