I like mine because I can store snacks, a waterbottle, my chain, 2 tubes and a gaiter (that I use when I ride in normal pants. Its a cheapie one but it has never come off the bike and theoretically, I can use it as a "fanny pack" because it comes with straps (though I have never done that...).

its a "bell" that I got from "Dicks sporting goods". it cost about 12 buck where if I got something at the LBS, it would be more like 27... I also have a bell seat bag that I got from walmart also a good deal where I put in my multi tool, chain break tool, tire levers and co2 cartridges... it also has never fallen off the bike and uses velcro to fasten it...