Some days everyone gets so burned of the work that ends thinking "Have I chosen my correct job? Did I choose well?".
But it's normal, and doesn't happens often if you enjoy what you do (even more if you work in what you have passion for, which in my case are computers and development).
And other times, you really enjoy working. Things go as planned, work gets done (even quickier than you thought), and you spend your 8 hours of work with pleasure and end having the reward of seeing things perfectly done.
Today was one of the later ;)
We're on phase 2 of a Windows Live Agent development, and having an initial estimation of near 40 man/hour work, we ended finishing all the big tasks and some smaller ones, almost without problems (probably my newbie skills on defining tasks and planning times for a 3-man team played out an important role too). Doing pair-programming with a colleage, helping and managing another one with his tasks, refactoring some parts, cleaning and optimizing others... We'e reduced the overall size of the project's code while greatly increasing the functionality.
While things not always go as planned, when you cross the line between "knowing something" to "having control of something", at least I feel I've done the correct decision working in an industry that requires much effort to keep updated on knowledge but also gives you the opportunity to create things, to help others (not only our clients ;), to do something useful...