If I am only carrying a couple things and around the city or a simple easy hike then I'd just take a shoulder bag since its not that heavy and its easy to access.
If I am actually going on a hike or up a mountain then I will always take a backpack since it distributes the load much better. While the access is more of a pain, the advantages out weight that disadvantage.
Also agreed. Hiking long distances, or climbing up & down waterfalls like I do a backpack really is essential. Also if you want to carry a lot of gear it certainly is the way to go. I tried a few before I settled on the Lowpro Flipside 400, it is the most comfortable backpack I've had (and most secure for your gear, although awkward to access it). But for short walks with only 1 lens, the shoulder bag wins hands down every time.