Slab is a simple, but very well designed wiki with a clear sense of purpose: it wants to tie together all the places your team currently stores information to become a single source of knowledge for the entire organization. The result is a wiki that lacks some of the visual niceties and tat of some other solutions, but which makes up for it in sheer usability.
They're threading a needle here. It feels simple and doesn't impose itself at all on your writing process, but it also subtly ensures that you're keeping everything well organized and referenceable by the whole org. That's a stark contrast to systems like Confluence, which are potentially very powerful if used correctly but using where using them "correctly" imposes such an additional upkeep burden they hardly get used at all.
Ultimately, a wiki is only as useful as the amount of infomation it contains. Slab is one of the few I've seen that my team actually seems to like to use. That's a winner today in my book, and I'm very eager to see what else they add as it develops.