My personal approach (which I don't consider to be the sane approach, or maybe even a sane approach) is that I want to learn a language either because I want to speak it with people (or read it), or because it's inherently interesting in some way -- for me that boils down to interesting grammar or word formation. So, if I wanted to learn a language now for reasons of curiosity, I'd have a few candidates ahead of Esperanto. From the ones I'm aware of: Lojban (due to unambiguity and very LEGO-like word formation), UNLWS (because it has so little in common with all the other languages), Japanese (because it appears to be extraordinarily compositional for a natural language).
Once again: my approach might be weird in many ways, and it's nice that we have people who generate internal enthusiasm in a large variety of different ways.