Ed, I bought this backpack about a month ago (for a lot more money). For your intended use, you could get by without the waist strap, although I do find it helps to support the weight a little more, if you really load up the backpack. I also own the smaller Flipside 300, and never use the waist strap on that bag.
I just came back from a 1 week trip to Glacier National Park using this backpack, loaded up with a 5D3, 7D, 24-105L, 17-40L, 100-400L, and 40mm f/2.8 pancake, along with a few filters, memory cards and a couple spare batteries and charger. Everything fit . . . the bag will fit into the overhead compartment of a smaller, regional jet airliner, which was a requirement for me.
Pretty impressive!
I'm really happy with my Lowepro Fastpack 250 and I love it. This deal makes me wish I could justify a second bag!
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