The only upgrade I've ever done has been to buy a set of aero wheels; Flo 30 front and rear. I loved them at first but then realized they didn't really do much to enhance my style of riding so they were definitely a waste of money.
I've never been one to upgrade parts and that kinda cemented my decision. I'll put in my preferred saddle, buy my preferred tires when the stock ones wear out and sometimes add my preferred handlebar style but to change non-fit items for a supposed improvement in performance is not something I'm into.