From what I've read (and experienced) is that battery life will be device-specific... as in, my Droid 2 won't necessarily get the same battery life as that guy's Droid 2. It basically comes down to usage. Personally, I use my phone as much as I want, keep the 3G, bluetooth, and GPS radios on 24/7, and have some things running in the background all the time (latitude, maybe a couple others), and I listen to music whenever I'm out. I can easily make it through a ten hour day with the regular battery. I don't browse the net, play games, or chat a lot, or any other activities that would require the screen to stay on.
The only way to really know if you'll need an extended battery is to use the phone how you want for a couple weeks, and see how it works out. If your battery is either dying or cutting it very close, the extended battery (or a second regular battery) would probably be a good idea.
(That said, this topic and many others that will probably be of interest to you have already been discussed ad nauseum around here, so you might want to browse the forums a bit.)
