I've got the Lowepro flipside 400AW
With it's waist strap I can take off the shoulder straps swing it to my front side and change my lenses while standing up (it's like having a shelf).
In it I have 5 lenses including my 70-200mm a 5d mark II with attached battery grip, a spare crop body. including a teleconverter and a flash, plus all my battery chargers. In the front I carry spare batteries, CF cards, collapsible grey card, pocket wizards, sekonic light meter, misc cables.
It also has straps for a tripod which I've used and ...the best part... it has the built in rain cover like many lowepro products.
I love it!
The 300 and up you can wear it frontside, buckle the waist and chest strap behind you, and hold a golf umbrella hand-free with the tripod carrier while taking pictures too
People look at you weird, but hey, you're dry