I've never used Overleaf, but for several years I used ShareLaTeX as my primary LaTeX editor. I've since switched to using LaTeX through emacs but I still regularly use ShareLaTeX's great documentation and if I didn't carrying my Linux laptop around everywhere, I'd probably still be using ShareLaTeX. Hopefully this new partnership won't ruin it.
Overleaf has its advantages (e.g. better git integration) but I haven't heard much praise for their editor. In my own experience, the Overleaf editor is slower and more cumbersome to use than ShareLaTeX. It's much harder to rearrange files and compile time to PDF is much slower. You also can't set your main doc to be in a subdirectory of the project.
I hope this partnership means we'll soon have the ShareLaTeX frontend on top of the Overleaf backend. That would be a truly powerful combination.
