I never considered upgrading sound on a car before. Yesterday I put on some tunes and was surprised how noticeably worse it was than my Fiesta. I'll definitely be upgrading now.
Do it! It's pretty easy to do yourself, and aftermarket speakers are a much more cost effective way to upgrade your car's audio. WAY cheaper and better than the factory "upgrade." A set of $100 speakers can easily provide a HUGE improvement in sound.
If you're going to DIY I highly recommend getting a set of trim panel tools, like these:
They're cheap, and will make removing the door panels much easier.