The Bug is DESIGNED for people in your situation. I have one, and it's a great bag. It's a good panier and it also makes for a good backpack. I've been in some pretty heavy downpours and the only time I've ever gotten anything wet was when the zipper wasn't fully closed. Still, if you're worried about it you can always wrap the stuff inside the bag in a plastic bag or get the waterproof cover they sell for the bug. My bike doesn't have fenders yet so the bag does get kinda dirty in the rain and wet weather, but it brushes off pretty easily.

I say go for it. It's high enough quality that it will last you for years.