This is a tough one for me. I tend to take pet projects just to the point where I know they're doable and feasible and it's just a question of performing the "brainless" work.

Instead of sticking with it and finishing them completely.

I think it's about prioritising what could have the biggest impact to your life and being honest with yourself as to what you want to get out of it.

I have now allocated time for "intellectual exercises" but I don't kid myself that I'm doing these for my career. And if I find a justifiable angle for it to benefit my professional career I make sure I put in place the steps to promote it on my résumé, blog etc.