This is a matter of debate, depending on what circles you're traveling in. If you're working in a "mixed environment" where some people are using OpenOffice and some people are using Microsoft Word, you're likely to run into problems that range from minor annoyance to show-stoppers very quickly. Some of this can be blamed on Microsoft's weird file formats, but this is true not only for .doc files but for the better-documented (if arcane) .docx and, in my experience, even with RTF. I'm in contact with a fair number of small-to-medium press fiction editors, and while a fiction manuscript is one of the simplest use cases you can imagine in terms of formatting, almost every editor I've talked with has complained about OpenOffice (and to a lesser degree LibreOffice) screwing up comments and revision tracking. I don't really want to use Microsoft Word, but so far I haven't found an open source equivalent for my needs that is good enough. (The closed source Nisus Writer Pro and even Apple Pages seem to do better, ironically. But that's not to say they don't have their own problems.)
I tend to think of open source's most well-established terrain as languages and server-side components where UI is not really a major issue.