Notability was a game changing app for me. When it appeared, I decided to finally buy and iPad. I've been happy with it ever since. Things got even better when they published their Mac version so I have all my notes across Mac, iPad, and iPhone. Now, with an iPad large enough to handle multi-tasking well, using split screen, I can see myself relying on the Mac version less and less.
As mentioned in the comments section, I'll likely have Notability open on 3/4 of the screen at any given time with a host of reference apps being swapped in and out constantly. This really looks to be a very helpful tool.