Great add-on! Loving the re-designed Wikipedia; the old design looks outdated and cluttered.
The table of contents looks beautiful and can be accessed from anywhere within the page, although it is not perfect, since sometimes there are a few subheadings are missing.
For the link pop-up, I wish there are options included for customizing it to your preference, such as being able to change the box size, thumbnail size, transparency, etc. The pop-out link icon also seems unnecessarily large and takes up space; it should be the same size as the text.
The search function is a handy little tool, though I still prefer searching on Google since I get more than just Wikipedia results.
One thing that I find annoying is that the "Install Wikiwand" text and wand icon (including the pop-up on hover) has been there for some time now, and many users have complained about this to which Wikiwand replied stating that this is a bug. It has been more than 3 years since the submission on UserVoice, and I am still experiencing this issue. Other than this, it looks like the advertisements that people have been complaining about are no where to be seen (with or without the add-on installed) from what I've seen so far, which is great.