I'm currently using a Slingshot 200, and I like it a lot. For me it fits my camera with a 16-80 lens, a 50mm, 10-20mm, 70-300mm and flash, no problem (i rarely carry all those, but they fit). I've also often got a film camera (OM4 or Bessa R2, so nothing huge), in the top compartment.
I've heard people complain of the slingshots being uncomfortable, but I haven't found this so far - but then I have only carried it around for a couple of hours. I'm not sure I'd want to do some serious walking with it.
When I am out for awhile I just use a normal rucksack though. Not as quick to access, and I do kind of worry about the lenses, but it does work pretty darn well (I think mine is around 40L, one large compartment, two smaller, and one slot for a laptop).
Not sure if that helps...