All of them are "good enough" for some users, and have compelling use cases. They aren't feature complete, but they all have small niches where their online nature gives them a compelling advantage over their offline counterparts. From this small niche they can grow "innovator's dilemma" style to be competitive with their desktop competitors.

Google Docs is the most obvious example of this -- I don't know anybody who uses Microsoft Word anymore. Obviously I'm in a little bit of a bubble that way, but they've certainly got a large enough user base to grow from.

Google Docs works while offline, and the same techniques could work on the competitors I mentioned.

In terms of lock-in, all of the services I listed are in the "underdog" phase of growth, where lock-in does more harm than good -- a good interop story with their competitors is more important than the benefits of lock-in. Of course, if they ever reach dominant status this might change, but at the moment, all of the ones I'm familiar with have good & prominent import & export options.