My background is with Visio (which is highly intuitive and very easy to use). Since there is no Visio equivalent for the Mac, I have turned to Omnigraffle.
Um, excuse me, but I know both really well, and Visio is not what human beings mean when they say "intuitive". Just because MS says that grass is purple does not make it so.
So you have a Mac. Welcome to a new universe of "intuitive".
To be reasonably effective, and draw diagrams in less than one third of the time it takes in Visio, you need to understand a bit about what Apple think "intuitive" means. Then it will be easy to use OG. Because it is one of the most Apple Standard-compliant apps available. If you know your way around the Mac, and you give up your ideas about how to do a Visio function in OG, and just learn a little bit about OG, you will be a very happy camper. Trust me, I am blissfully happy camper, and I cringe at having to do something in Visio.
Some people do not like to be told, but th ebiggest obstacle to the learning process is the confidence that you already know the subject.
Learn each of the tools and inspectors, they are very powerful. And they have levels of power.
Then there is the next unvierse, of drawings you can create in OG, that you cannot even dream of in Visio. Like this, for one, and I am not an artist, ok ?
Example Technical Drawing
That's not meant for reading, just for oohing and aahing. Do click the blue glass buttons.