Actually, I use a fanny-pack like bag (as a fanny pack) when hiking or skiing. Its not of the type mentioned in the original post. I think its a Mountainsmith and I put a pad insert in it. I like the water bottle holders on either side of the main bag, but sometimes worry about getting gear wet. Hasn't happened yet, the material is adequately waterproof.
I also have an everyday bag that I use as a shoulder bag, but it has a fanny belt built in (tucks into a pocket on the back of the bag). I haven't used that feature yet, but I did think it was a positive feature when selecting a bag.
I dislike lens cases in general (excepting the bag/cases for very large tele lenses, such as the 600/4). But for my M lenses (say an extra 50 or a 28) I much rather have my bag with room for said lenses.
haring, what kind of lenses are we talking about? How many? Is this primarily for wedding jobs?