I experimented with them (also with capnproto) at my last job for a usecase involving dense numerical data where being able to randomly seek within a dataset would have been really helpful for speed reasons, but found that as compared to protobuf, these formats were unacceptably bulky (lots of extra padding for word alignment, etc.), and the added cost even just to read the extra data from disk mostly negated the savings from avoiding the explicit decode step, plus would have had significant implications in terms of storage cost, etc. I ended up writing a custom wire format that allowed for seeking with less wasted space.

Seems like a neat idea, but as another commenter said, the usecases where it's the best choice seem pretty narrow.