I have been using the web version on Linux box for almost half a year (because I don't own a Mac right now) and found it to be acceptable _only_ as a companion to iPad or iPhone version. It's not usable standalone at all (for example, you cannot create recurring tasks, or even move a project into a folder, or even _create_ a project in a folder). I hope they improve the web version, but the development doesn't seems very active[1]
That said, I've tried everything else, including Todoist and even org-mode, but found myself come back to OmniFocus in the end since I found Perspective to be essential to my workflow (even though web version doesn't support Perspective at all, at least I can use it on my iPad). Org probably come closest to fully replacing OmniFocus, but syncing between a Linux box and iOS devices has been... not fun.