i find that i have many many knitting bags. i have a bunch of handbags and totes anyway and i use them all, even occasionally, as a knitting bag. it almost entirely depends on what project i am working on, as well as where i am going (if anywhere) and how long i'll be. so, i'll describe my main uses for my more popular choices...
1) the all inclusive: i have this pretty big tote, made out of recycled detergent containers (a kind of capri sun material). it is ver boxy, rigid and sturdy. in a corner of it there is a diagonal divider. i think it is intended to protect a wine bottle. i have put down inside of it a box that a bottle of scotch came in and in that i carry a whole bunch of needles, a pair of scissors and a metal ruler. i store a bunch of my rarely used ones in there, since it's roomy. in the main (big) part of the bag i keep tons of yarn. not like seven skeins of identical yarn intended for a sweater or something, but about 5 gallon sized ziplock bags each with similar weight leftover yarns, novelty yarns that i just happened to buy, or what ever i pick up along the way. this way, i can pick a bag and have enough scraps of similar yarn to make something up, or just knit stripes to felt later. there is another smaller side pocket that has room enough for needle holders, a stitch counter, cable needles, finishing needles, etc. in yet another, even smaller pocket, i keep some buttons and beads, and a regular (hotel style) hand sewing kit. i can easily throw my most current project on top and my needle wrap down the side or replace the liquor box with. i use this for road trips, weekend trips to friends, car camping, etc. not suitable to bring on an airplane or anytime you have to carry much. i end up with this one often when i'm going to be away for a couple days and am about to finsih up my current project and think i might want to start something else before i return home... (but am not so sure what yet)
2) hobo bag. i have a suede hobo bag that i got at the gap about a hundred years ago. it has a shape that is very popular with knitting bags i see at yarn shops, the kind with the round bamboo handles (though mine are longer and leather) i use it as a regular handbag too, but when open it has two equal sides -so knitting project goes in one side and my otherwise minimal neccesaties go in the other. it is also big enough if i have a pattern book to slide into it. the coolest thing about this is that it has these two outside pockets -like a kangaroo pocket- that is perfect for keeping a ball of yarn in so that it feeds yarn perfectly, but doesn't comeout and drop to the ground. i can litterally walk down the street with this bag on my shoulder and knit at the same time.
3) fishing basket. i like to use fishing baskets with lids at home or at a knitting club for the same reason as above, except they sit nicely beside you on the floor. i keep a tupperware or chinese food carton with a different ball of yarn within each, inside of the fishing basket and feed various yarns out of the corners of the basket. i keep another little plastic wallet-thing i have with smaller notions in there too. also good in the car.
4) the needle wrap. this for all of my straight needles and the denise interchangeable kit for all of my circulars. easy storage and portable and slips into most of the other bags.
5) travel bag. i like to use a messanger bag. mine is a black, flimsy (by this i mean, not heavy or "high-tech") lined, felt bag, that is pretty non-descript. i keep meaning to embroider it or make a felt coat pin for it. i got it at urban outfitters about the same time i got the gap hobo bag. like messanger bags are wont to do, this one has two large main compartments one of which i can typically store up to 7 skeins of bulky yarn (it'll be sorta stuffed, but not overly). in the compartment behind it i'll have a pattern book or two -whatever i am currently obsessing over or have just purchased- loose patterns and/or a zine or more. the front part, that flaps over, is itself a large zippered pocket. in it go needles, notions, etc. the back, too is a large zippered pocket, on the outside and i'll keep this free for my wallet, cell, etc. if i'm not carrying a separate handbag (which i don't like to do so much). my needle wrap will fit into the 2nd internal compartment that i keep books and patterns in, but the denise interchangeable kit i'd keep in this outside back, zippered pocket. too heavy for the flap pocket.
hope my over-explanation is interesting to you!