I was very excited to buy a Mac Mini the other day. I was very disappointed to learn over several weeks that, despite what Paul Graham and DHH and others say, OS X sucks for hacking.
I experiment with a number of languages, I pull in many different libraries, I use various different tools and scripts and programs from the command line. Frankly, installing any one of these things onto a Mac is a pain.
In Ubuntu, I simply do apt-get install. On a Mac, I have to download a tarball and compile it (their package management systems have many fewer packages, many of which are out of date). If I wanted to compile things, I would use Gentoo. Actually, Gentoo is much better since I think the portage package management system is very well-developed, up-to-date, large, fast, handles dependencies brilliantly, and has the option for some binaries.
As for the general use of the Mac: if you want to do anything that is not the default, good luck. Learning all of the different ways you can do something non-standard on a Mac is like learning black magic. You have to Google around to find out the secret incantations. For example, to ask for the boot menu, you must psychically know to hold down ALT as it boots. The other day, I wanted to test out how well my site worked with Flash off, only to discover I couldn't. I found a site with some complicated instructions about how to go about disabling Flash on OS X. I just gave up and rebooted into Ubuntu.
Basically, hacking on OS X is exactly like working with linux 5 years ago, when I had to be an expert on esoteric systems to do what I want.
Just do what I do: dual-boot into Ubuntu. I boot into OS X when I want to use iTunes, watch TV/movies using the cool Mac remote, or develop iPhone apps.