For some reason NSButton doesn't use subpixel antialiasing for its label like other AppKit controls such as NSSegmentedControl.
Here's an example:
I noticed this right away and I found it really annoying.
In order to fix this in my work, I created a subclass of NSButton which overrides the drawRect method and enables subpixel antialiasing for drawing the label.
Here's the code:
The result can be seen below. Both buttons were created using Interface Builder, but the top button has its class set to a subclass with the drawRect override. The bottom button does not. The improvement is easily noticeable:
Curiously, Apple seems to have done something like this for its own applications. I checked out buttons in System Preferences, and their labels are drawn with subpixel antialiasing. I wonder why this isn't the case for apps that people make using Interface Builder? Odd...