Ithkuil, an invented language (for humans)

as intriguing as it might seem at first, the Sapir-Whorf hypothesis, which suggests that the language you speak is closely related to the way you actually think and that enhancing the language will enhance your cognitive abilities, stands on weak grounds, and for all who, like myself, are intrigued by it, I'd suggest an insightful reading: The Language Instinct, by Stephen Pinker.

In this brilliant book, you will find, among other things, good arguments that might lead you to reconsider the Sapir-Whorf hypothesis.

That said, I still find conlanging a very fine and amusing intellectual endeavour and among conlangs I find Ithkuil to be one of the most interesting examples (my nick is just a tribute to it, I am in no other way related to it).

http://en.wikipedia.org/wiki/The_Language_Instinct

The interesting question is why the constructed languages never gain wider acceptance. The only example of (semi-) constructed language that became first language for a lot of people I can think of is Ivrit:

https://en.wikipedia.org/wiki/Revival_of_the_Hebrew_language