I use both – single strap backpack and belt setups – though I use the belt + cup setup a lot more than I do the backpack for work. Works great while on the job (I shoot in ultra packed nightclub / event conditions and it’s way easier to squirm my way through crowds with the belt). It’s fairly light (with one cup / lens) and very low-key compared to the backpack.
With the belt setup + 1 medium cup, I can hold:
a Canon 5D
2 lenses up to the 85 1.2L size (or 3 if the lenses are near the 50mm f/1.4 size)
one 580ex flash + 3 sets of AA batteries (@4 batteries per set)
3 sets of 5D batteries
up to 6 CF cards
A flash cord (the size of a standard telephone cord)
a wireless remote (ST-E2)
The velcro linkages are pretty heavy duty but I secure the linkages with plastic tie downs just in case. While I’m not shooting (and everything is packed in) I sling it over my shoulder exactly the way I would my single strap backpack.
Works great for me since I usually carry no more than 2 or 3 lenses (all primes) with me anyways, unless I’m off on a long trip… in most cases tho, I prefer using the belt over back pack or side bag… thats me though. It’s not for everyone so definitely try it out before you buy it.
Single strap backpack —> lowepro.com/Products/Sling_Bags/
Modular belt system —> tamrac.com/welcome.htm