The Panasonic "shotgun" is a step up and much more directional than the built-in mic, but I'd recommend something better if you need a directional mic--like an AT897. Sometimes, a directional mic may not be what you want, like when you're filming dancing and you don't wan't the sound quality changing as you reposition the camera. In those cases, the on-camera mic will do a good job. For interviews, a directional mic is a must. For the ceremony, lavalier mics are usually a good choice (unless you can enlist the best man to be boom operator ;D ).
I noticed that when I use the built in mic, it picks up what's behind it better then what's in front! If you haven't purchased the MC100 yet, you can get superior mics for the same cost or even less at times. Rode makes a great on camera mic that goes for around $100, I paid $150 for my MC100. It's a decent mic if it's all you have, but I will always avoid using the built in mic personally.