I hang out at StackOverflow and don't use boost. I will add my reason, because it is one not mentioned yet.
Boost has many great ideas, really. I like to look at what they have done and try out new things and ideas. They are great, because it is breeding ground for many C++ improvements.
But the boost is a very unwieldy beast for many reasons. One of the reason is that they need (want to) to be compatible on virtually any compiler with any quirks. As a result they need to employ many tricks, such as MPL to pull it off. For example (a long time ago) I wanted to use their shared_ptr, getting it to run meant I need the sources and libraries of what felt like 90% of boost. I ended up writing my own; 50 readable lines of code. (My requirements where stricter, like no weak_ptr or thread safety.)
Often you need a really small subset of boost, but integrating the entirety of boost is just not worth the hassle.
Edit:
Just to make is clear, since it appears not to have come over clearly (i.e. downvote). I use do use third party libraries. But in most cases, all things being equal, integrating a third party library or boost, the other third party library is quicker and cleaner. The remaining is done in "2h" finger exercise. I do take a very hard look in the build it or buy question.