SimpleDiagrams is the Goldilocks of Mac graphics software. Omnigraffle (which I own) sits on one side as too large, too complex, and too expensive for knocking out quick diagrams. And Delineato Pro (which I own) sits on the other side as too simplistic, too unreliable, and rarely updated (albeit cheap).
SimpleDiagrams comes loaded with many vectors sporting that "hand drawn" look. And you can import a bunch more for free. And pay for more for a few bucks if you like.
Even better, you can easily import your own PNG or SVG files into the program (even those that don't look at all like they are hand drawn!). You can organize and tag them, AND very easily export your custom collection for import into another of your Macs. Sweet.
Adjustments to colors, resizing, and all the fiddling you want to do are simple and intuitive. There is a straightforward online manual that explains the basics.
Two minor quibbles regarding font selection: I'd like to set a preference for a default font, and want the font selector to access the system font collections so that I can quickly tap into my carefully honed fonts.
But these are nits. The program is easy to grasp. Fun to work with. And a great value. I will be diagramming more with less hesitation because there is no reason not to!