Whats you bag?

I got thinking about camera bags today, over the years I have had loads, my favorite was a Billingham 450, it was a decent size, well made and like the Tardis (bigger on the inside) the amount of kit I could get in was incredible! On a good day I could get 2 film cameras with motor drives, 8 lens, a Metz hammerhead flash, 2 boxes of Coking filters, plus loads of film etc etc etc.
You may notice I used the past tense... the reason is I no longer have the Billingham you see, good as the bag was, I'm not getting any younger, and with all that kit in it the weight was doing my already dodgy back in. At my age I really needed a rucksack type bag, so when the opportunity came up to swap the trusty Billingham for a rucksack, I jumped at it.