Recs.
Updated
SpecsUpdate
Pros
Pro Development is supposedly going to begin again
Originally Haiku was based on the BeOS which has since been discontinued. The alpha release was in 2009, and the most recent alpha build was compiled late 2012. There are still regular updates on the official website, and development hasn't stopped.
Cons
Con Small community
It is important when developing to be familiar with tools that other developers use. You can make any utility in any language you feel like, but if it's in an esoteric language that no one can read targeting a small platform that no one uses, then it was just something you did as a hobbyist, not as a developer.
This is not to say that Haiku isn't a great operating system to hack around on. Just don't delude yourself into thinking you're doing it to get familiar with tools that you need to know to be a better developer.