When you get into Pro Bags, especially for the longer lenses, you can plan on spending a hefty chunk of coin on them. With the acquisition of my 600mm, I've forayed into "expensive". There are two bags that I hope to get in order to carry my 600mm f/4. One being the "expedition class" Lowepro ProTrekker 600AW, which is a heavy duty expedition type backpack with plenty of room for other lenses. This lens bag, if I used it, would probably hold ALL my lenses in my photography lens collection - and I would end up looking like a turtle on its back - helpless and unable to get up without help.
The Lowepro ProTrekker 600 AW is labelled as such.
The other being the Lowepro 600 AWII which is a single-lens backpack type lens bag that allows for me to carry a 600mm lens and the Gitzo tripod (on the acquisition list). Both are extremely well-padded naturally considering the expense of the super-telephoto lenses that you'll be carrying with this bag - It will carry up the the 800mm f/5.6 lens (of course, with camera removed). The 600mm will allow ample space to fit the camera on so that all you have to do is remove the lens from the bag and shoot.
This will be my day-to-day camera bag for the 600mm as it will enable me to bring it anywhere I need to go instead of lugging it outside of its case and potentially having it damaged - let's say on the off-chance that I have to stop by a grocery store or mall on the way home from shooting. Most normal people would drop the lens off at home first, but then again...who said I was normal?
a) depending on the lens you have, the largest lens that you have in your lens collection will dictate the bag that you get.
b) the type of shooting you do dictates what you take along with you. If you are going to go on a trip to Banff that is essentially going to last a week or more, you might want to take the LowePro Pro Trekker where you can have maximum flexibility of lens selection. Whereas if you intend to only go for a dayshoot, the Lowepro 600AWII may be the perfect lens bag for the day-hike if you're aiming to shoot wildlife for the day.
The Lowepro ProTrekker will be perfect when I add the 300mm f/2.8 VRII to my collection of lenses. as the 600mm f/4 and 300mm f/2.8 will go in side by side and I will be able to add in a 14-24 and potentially a 24-70mm at the end in order to do landscapes at the same time. Also, the Lowepro ProTrekker has a sleeve in which you stick two legs of the tripod you have and then tie-down the other leg so that your tripod is secure as well.
Bags protect your lenses and your cameras, so it is important to get one that will do the job properly. After all, you put a lot of expense into the lenses that you have. Isn't it worth it to spend some more to protect your investment from breakage?