I think it's still worthwhile even if you don't have "success". I spent about two years steadily cranking away on a side project almost every day. At a certain point I realized, "Whoops, this isn't going to work," and just stopped. I had hit a dead end. Was that two years wasted? Nope. I taught myself a huge amount of stuff about data structures, functional programming and application architecture in the process that I wouldn't have had the freedom to try in my real job. I learned a lot from the mistakes that lead to me coding into a dead end, about how to validate ideas more quickly and cheaply. It was a "failure", but the lessons learned have proved to be hugely beneficial in other projects. The payoff was far greater than just personal intellectual satisfaction.