I've now used it for two months and it's the best PDF reader I've experience for reading academic articles. It's snappy and reminds me of replacing my web browser with Chrome when it first came out. Can easily add native PDF annotations and search through the existing ones. (And it saves them fast when you close the file. Competitors either do this slowly or, like Skim, use a non-native annotation format that can't be read by other PDF readers.) Draggable tabs. Plays nicely with Zotero. The UI for "find" displays a lot of info intuitively. Everything is just well designed. I haven't yet run into a limitation on the free version, and upgrading to pro is only $20 anyways.
It used to crash from time to time when it came out in late 2017, but this appears to have been fixed as of Feburary 2018! (That's why I'm upgrading my review from 4 stars to 5 stars.) If there are old reviews complaining about crashes, keep that in mind.
I prefer it over these other PDF readers on maxOS that I have tried: FoxIt, Preview, Adobe Acrobat Reader, Skim.