When I see someone towards the middle of their career who is still in the thrall of C++ metaprogramming, it excites in me a mixture of genuine sympathy and condescending pity (and if they're a coworker of mine, righteous wrath). I am glad I went through that phase in my late teens and had the nonsense shook out of me by my first professional experience as a programmer.
It's fine to pursue this as fun for its own sake if you are under no pretense as to its practical utility. But the danger of having your fun it in a bread-and-butter language like C++ is that you will be tempted to let it infect your workaday coding style. That's one of the perpetual dangers of hiring bright, overeager C++ programmers at the junior level.
Boost is a minefield. Its less booby-trapped libraries have made it into the next C++ standard library, but the more hardcore indulgences in metaprogramming like Spirit deserve all the scorn that is customarily heaped on them, if not more.