I have the Tamrac Velocity 9.  My somewhat standard kit is a large DSLR
(Nikon D2x) with the 18-200 zoom, a 200-400 f4.5-5.6 zoom, and a 12-24 zoom.
Plus extra stuff of course that goes in the pocket on the back. It's a tight
fit, however I usually take the camera out anyway when I'm using and hang it
off my neck.

The shoulder strap has the standard kind of adjustment clip that you see
with other nylon straps, i.e. the strap goes into the clip, over, and back
out.  So you just lift the clip to release the strap, and then pull on the
loose end to tighten back up.  Most of the time I use the bag I don't change
the adjustment though.

I find that the bag can be uncomfortable to wear for long times (say over
two hours).  The strap tends to dig into the side of my neck, and also it
restricts my breathing.  It does come with a belt strap also however I have
found that that is not very helpful.  In the future I will probably use a
regular two-strap knapsack instead of the Velocity, even though I really
like the convenience.  I guess in part it depends on how much you like to
change lenses or grap for other things such as filters.

Also BTW the Velocity is part of the Tamrac "system", so one other thing I
got is a filter case which uses a velcro strap to attach to the side of the
Velocity.

And one final comment, there are no external attachment points for things
like tripods.

HTH