Is changing careers a good idea?

Do frequent job changes sabotage your career? Or are they a springboard to a fulfilling career move?  

Did you start out as a salesperson, and then discovered you'd rather be a teacher? Or you were a finance whiz kid but thought you'd make a better documentary film-maker?

Does it make more sense to stick to the profession you started out with, or switch to something you've discovered gives you more fulfilment and satisfaction? Is the pursuit of happiness, career tailored to your needs, a worthy pursuit?

We invite those of you who have switched careers midway, or have an opinion on the subject, to share your views.