I've a mini-trekker AW. I've taken it out riding but it's not ideal. I can geta bladder in, but it's not made for a bladder and I do worry about it a little. MOre than that though is the faff of removing the bag, then undoing all the clips and straps and whatnot to get the camera out. On top of that, it's a "traditional" bag in that the outside opens, so the bit lying on top of that dog turd you didn;t spot is the panel that goes against your back. Nice.
Not sure what camera bags exist where the back panel opens, but I'm sure there must be one. Check Seb Roger's blog, if he hasn't posted up come quick reviews of bags, he's posted inks to people who have