I don't get close to 5gb even on my iPhone, mainly because I use WiFi whenever possible. I assume that this will be the same with the Droid. But 5gb is a lot of data for sure....
I've been really lame with my Storm since it's so cumbersome to use the browser and other items. The most I ever hit with it was just over 1Gb. With my Voyager, due to the amount of tv I watched on it, I regularly hit between 2-4 GB, depending on whether I was traveling for work and training or not. It really wasn't hard to do at all. That phone had me on the net all the time because it was such a 'dumb' phone and so easy to do what I wanted. I loved it, but alas, I had to try the God-Phone to give it a fair shake (but that's a whole 'nother story).