Heh
My current favourite backpack is a real cheapo from Amazon - this one:
Bestek Camera Bag BTDB01 02 03 05 06: Amazon.co.uk: Electronics & Photo It looks like sh*t and attracts zero attention but is actually surprisingly well made and comes complete with a rain cover (which works).
The only drawback I found was that the padding in the camera compartment is soft, resulting in anything you put in the top pushing down on your kit. I have got around this (bear with me) by buying one of these:
Seal Fresh Storage Container / Sandwich Packed Lunch Box / Meat & Poultry Storer with Lid - Size 140(h) x 210(w) x 300(d) mm - 7.5Ltr : Amazon.co.uk: Home & Kitchen if you remove and discard the lid, then use a dremel or something to take off the raised lip that the lid seals on to, you can put it into the compartment within the bag, then the padding goes inside and your kit inside that.
It might sound a bit Heath Robinson but it works really well, provides a bit more protection (the open side of the plastic box is against your back when you are wearing it) and adds effectively zero weight to the total. Result - a discreet but very usable backpack with more than adequate protection for a body and up to five lenses plus assorted chargers, cables, spare batteries and the like. You still have to take it completely off to access your kit, but it is more secure as a result.
Mine has been with me this year to Florence, Pisa, Siena, Chianti, Sorrento, Positano, Pompeii and the Isle of Capri and has proved itself well. The biggest drawback I find to a backpack (other than the obvious access issue) is that some museums and art galleries ban them. My answer is simple; one of the side pockets contains a small folded nylon "shopping bag" that I can pop my "sandwich box" into and stroll around with, thereby avoiding leaving kit in a potentially insecure left luggage facility. It's not elegant but it works...