Yeah, that's what I thought in the beginning -- code formatting is all about indentation, which is handled by the tab key. But it turns out, it isn't. I only noticed after using clang-format for some way. Code formatting ties up part of your brain, every time you type a character, you have to ask yourself "do I need to insert a space", "do not need to insert a line break", "should I join these two lines again", "does this comments need re-formatting", "should I line-wrap this lengthy expression differently", "how do I make these lines line up nicely", etc.
With clang-format -- none of this. You type in the content you want, you insert spaces where absolutely necessary, you save, and when you look again, the code looks nice. And you can relax in the knowledge that this formatting is a projection -- there's no "other way" that maybe might look better.
Also, clang-format doesn't indent namespaces the way the tab key does, and it doesn't make errors. Clang-format is the complete separation of form and content, and you only have to care about the latter. Think of it as latex: You type the text, and it will look well-formatted without requiring any manual input. You may think that latex isn't necessary and that manually formatted text will look just as nice -- and you're probably right, in almost all cases manual formatting would look nice enough to do the trick. But there is a fundamental difference between latex and manual formatting, and it's good to have it.
So: Yes, this is worth it.
Regarding various versions of clang-format: There are (of course) also many options you can set, although I just go with the default, which is very good. Yes, there are probably difference between versions (improvements, e.g. for handling template arguments). I'm sure we could settle on a standard version, or a standard set of options.
-erik