Certainly an exciting step, although the program here was pretty excellent. Good supervision, full funding for the entire duration, topic of my choosing (I wrote parts of the grant).
The main reason I'm leaving is that Mech.Eng. PhDs just seem like such a conservative bunch. Most end up either in tiny, provincial ventures (think utilities companies, for my field) or as middle managers at huge companies (where I live, car manufacturers). They then are, for all intents and purposes, managers and not engineers anymore. The title is the ticket to a cushy life. Academia was never an option; the kind of work professors and researchers do seems horrendous! You're chasing grants and need to manage such a wide range of the most frustrating but also mundane issues.
Here's to hoping a software development career is more fruitful and fulfilling. A life-long IC position seems attractive. You get the benefits, but little of the fuss.
It seems like PhDs are sought after (if they are) for their frustration tolerances and project management capabilities. Maybe I didn't have enough of either. Then again, I'm absolutely convinced many more would quit prematurely if they had attractive alternatives. A lot of mechanical engineer PhDs don't, but thanks to the software world and my passion for it, I do.