I imported a report I wrote in Overleaf, and I must say I'm impressed.
Rendering is not automatic, but much quicker and of higher quality. (selectable text vs blury images in Overleaf) In Overleaf you sometimes spend quite a bit of time waiting if your long equation in a long report turned out correct, only to find out someone else made a syntax error.
Error reporting is also much better. In Overleaf an error in your bib file would usually generate an opaque error that leaves you hunting for the missing bracket. It remains to be seen how this works out in a collaborative setting.
While Overleaf also let you jump to the code corresponding to the rendering, ShareLatex goes both ways. Neither of them are very reliable, but it's a useful feature when it works.
I've not tested it, but it seems ShareLatex allows you to choose the LaTeX engine and keybindings. So Vim/Emacs users will probably like this.