I have been using NoteTaker 4 and have found it to be a great digital notebook. I do wish that NoteTaker 4 had more customization options (more control over the cover, tab color control, more skeumorphism 
, etc.).
This is a screenshot sample from Aquaminds
You may have seen this review:
In the past, I used Circus Ponies Notebook and really enjoyed that program. Unfortunately Circus Ponies Notebook is no longer being updated.