I’ve spoken Esperanto, Lojban, and Klingon. And as I’ve posted elsewhere, I have a soft spot for Interlingua and Interglosa.
There are studies, but the numbers are hazy. The numbers mentioned here though are congruous with what I know. Esperanto would be in the hundreds of thousands; Klingon in the hundreds, Lojban in the much lower hundreds.
Long-term advantage as a world-language? Even Esperanto has pretty much given up on that.
Benefit that arises because of widespread use? Of course Esperanto has more usage and literature and community. Though English has even more. And what counts as useful really does depend on what you’re after.
In fact, I remember coming across an argument on Usenet along these lines (yes, I am that old), where someone was saying that you can get much more quality interactions in Esperanto than in English, because it’s a better-quality, self-selected community. Yup: the retort came quickly that the interactions in the Klingon and Lojban community are even better by that metric.