It's not cheap, but the 3D walkthroughs help with visualising what the finished result will look like, including wall and floor finishes, and with furniture. I think I bought it from the Mac App store rather than their website.
Excellant point. It's pretty easy if all you do is drag and drop stencil elements but has a pretty steep learning curve to get proficient. Adding to the problem is their insistence on using interface items that differ from what one might standard conventions; and some of the differences are maddening, such as auto saving changes as you make them under the existing file name so you lose the earlier version unless you first export or duplicate the file first. There is no "Save as..." If you rename the file it changes the name in finder and saves it under the new name so the original is lost.
I like Omnigraffle but it's user interface is not well thought out, and now V6 is completely different.
I would recommend you OrthoGraph if you need precise floor plans. It is a great app for iPad.
It is easy to use and after you have created the floor plan,and have furnished your house you can walk through it in 3D.
Here is the website of the app:
I've been happy with MyFourWalls ($30 on Mac App Store) for drawing out floor plan alternatives to evaluate room sizes. I would have preferred to use OmniGraffle but it doesn't show/calculate area of rooms.