Well I've used this app on a daily basis for a full week (I need a good PDF reader/annotator for work), and I feel I've come to some pretty solid conclusions.
(I've also used Upad, iAnnotate, PDF Reader, Notability, and a few others for long periods of time, and I think I've got a pretty good feel for what I need/like/want.)
THE GOOD
Seamless page turning. (Notability's slow rendering on annotated or image intensive PDF's is horrible. iAnnotate is great with rendering, but if you have highlights or notes on the page, it won't read your page flip touch and will instead move your notes or call up the highlight edit box.)
Great highlighting (I love how you can highlight in nice straight blocks).
Nice additional underline features (I use the "squiggly" quite often, and it really catches the eye.)
Nice UI (I prefer the "shelf" look, but it's got a very "iOS" look to it, which is pleasing to the eye as well.) I like the tabs, and the single tap on the page to bring it up or hide it is very nice.
Dropbox support, which is critical for my needs.
THE BAD
Rendering isn't perfect. If there are images or highlights/notes on the page, it will take a second to render your annotations. iAnnotate was better at this, and I hope that a future update will fix it.
It may seem a small thing, but that gray circle with a triangle in the middle, located in the upper left hand part of the page, is very annoying. I spend a lot of time reading material with white backgrounds, and it's an irritant. I also don't think it serves any purpose since tapping once on any part of the screen does the same thing.
The handwriting is too big. I like the zoom box feature, but the actual ink needs to be much finer. It's probably a 3-width on Noteshelf, which is way too big for writing important notes on paperwork. The handwriting feature is pretty much useless, so I have to use the text box. The text input is great, but a bit time consuming if you don't have a bluetooth keyboard handy.
Organization: I wish you had the option to sort your files in whatever order you want, like icons in iOS. You can only sort according to name or date, but because of issues with my line of work, I need to be able to sort them according to different criteria. I understand you can make folders to group similar files, but I need even more organizational flexibility. If I could drag the file to where I want it and be able to keep it there, it would be great.
In conclusion, this will be my daily driver for a while. I'm very satisfied with it, except for a few little nits I picked above. But in general, I think it's the best one out there at this time.