Since I just left a comment while this app is an macUpdate Promo, I thought I would also post part and change parts of it to make it a review of my experience here where it is more permanent.
I have no connection with the developer of iDocument 2 - just wanted to get that out of the way first. :)
Since I had bought iDocument in the past for a previous version, I sent the developer an email asking if there was upgrade pricing. I got a prompt response, a great upgrade price and instantly received the license and I am putting iDocument 2 to good use. So, my experience is different to what some others have stated here.
I read a comment that iDocument and the updated version is just a big-fix update you have to pay for. Was this for an iDocument 2 update to an iDocument 2 newer version? In my case, I upgraded from iDocument (guess you could call it iDocument 1) to iDocument 2. My experience is that it is indeed a very different version. The interface is completely different - very clean. The layout much easier to work with. And the response speed is very fast. As an academic, I have thousands upon thousands of documents. With iDocument 2 I was instantly locate a document that I had been hunting around for a couple of months ago. I was not able to do this as effectively in the previous version. So the updated version works well for me. :)
If you are after a similar app, there is also "Yep!" (which I also got through a MacUpdate promo).
Anyway, just thought I would share my experience. :)