Another Camelbak Flashflo user here, I use it on most runs as I like to sip all the time and get an annoying dry mouth otherwise. It takes approx 1.5 litres of water plus space for keys/gels/phone/stuff. It doesn't bounce at all but I have it cinched down as much as I can and still breathe. Because I am short (5ft 4) it feels a little big on my back initially but I forget about it once running.

I don't think it sloshes that much, but I do suck all the air out a Camelbak bladder before doing anything.

Have only just started experimenting a bit with gels - like the High5 lemon Cola one (its caffeinated), the PowerBar Vanilla one was a bit yuck though, I have a strawberry one (I think) to try. I don't want to use them all the time but I thought it would be good to carry one or two in my Flashflo as my long run mileage increases. When I was doing eight mile runs at the beginning of the year I would start to feel like I was in need of some calories (I run at lunchtime before lunch) and gels could be a convenient way of doing this. Also thought they would be useful as emergency rations on hikes/long mtb rides.