bobcatt More people know about Klingon than esperanto, but Klingon is practically impossible. There's no way that more people speak Klingon than esperanto (maybe if you count people who think they do but haven't actually attempted conversation yet). I've tried learning both. Anyone can learn, speak, and read esperanto -- it's designed to be that easy. Klingon is designed to be nearly impossible to learn. Just look at its sentence structure and wince.
One thing I'll say in favor of esperanto: it helped me understand the subjunctive case in Spanish.