1. The low latency feels really good. It's snappy.
2. Smart Jump is great. It seems to target the figure caption though, and depending on zoom level / window size might leave most of the figure out of view. To be fair, the figure links that publishers bake into their PDFs often have the same problem.
3. Smart Jump to figures worked reliably in the scientific articles I tried. Smart Jump to bibliography entries and equations did not, although it worked in the tutorial PDF. Journal PDFs have terrible formatting, so this is understandable. One test case I tried was using a wingdings-like font for the citation brackets; what looked like [13] was stored as @13#. So smart jump works best with well-formed PDFs, but well-formed PDFs also usually already have links embedded.
4. The helper window opened behind the main window (for me, anyway), which is a little awkward.
5. As I was reading, I found myself wanting Smart Portals more than Smart Jump. Same automatic cross-reference detection as Smart Jump, but defining a Portal instead of a link.
6. The view in the helper window can't be zoomed or panned, so unless it's kept at the same width as the main window was when the portal was captured, the helper window tends to crop out part of the captured content.
7. The keyboard-centric navigation is well done and a unique selling point. The Windows file picker feels out of place with this though; I'd expect the file picker to work more like the bookmarks list.
8. I found myself liking shift-o more than I thought I would, more than tabs. It would be helpful to be able to clear entries from the previously opened list though; mine is now littered with test files.
I think I would mainly consider Sioyek when reading book-length PDFs, as some books have a lot of distant cross-references and Sioyek's features could save time there. Probably not article-length PDFs since their brevity makes them easy to navigate by default, and they unfortunately tend to break smart jump.