In Preview under Mountain Lion, it's a snap to fill out forms. The new version analyzes your PDFs and recognizes document elements like checkboxes and radio buttons, so you don't have to do anything special—just open any PDF and start filling it in. To me, that's pretty much worth the cost of 10.8 right there!
To make this work, Preview needs to be in Text Selection mode. So click on the Show Edit Toolbar button at the top of the window.
Then make sure the Text Selection button is toggled on (it'll be blue, as below).
Just like always, if you don't see those buttons, use View > Show Toolbar to unhide your toolbar, or choose View > Customize Toolbar to add the missing buttons back in.
After that, there's really nothing more to know. Preview can detect checkboxes for you to click in…
…or elements like underlining for you to type on.
In my testing, this has worked pretty darned well. I've tried several different forms from the government and other sources (including the one I goofed around and made in Pages above) with no issues. Why do I need a printer again?