The SKS Energy Bag is nicely made and easy to access, and it's pretty cheap too. Triathletes have been using top-bar-mounted energy hoppers for years, and they're becoming more popular among road riders too as they give you a handy place to chuck a few gels or your phone.
The Energy Bag has a semi-rigid construction with an overlapping lid that secures via a Velcro tab at the front. It mounts with two straps; one round the head tube and one round the top tube. Most other bags of this sort have two top tube straps, but I didn't find the Energy Bag to be unstable.
Once it's on the bike you can fill it with stuff. It'll fit about five gels, and it's just about big enough for a smartphone; my HTC One X is pretty much the limit of what will go in. The lid closure is okay but the semi-rigid nature of the bag means that it sometimes takes a bit of fiddling to get the velcro to catch; not an issue if you're just carrying supplies but you won't want your phone jumping out over the speed humps. Side-to-side stability is okay; although it's fairly easy to knock it off centre it's eaqaully simple to shove it back again.
It's not too intrusive in use. Standing up on the steep stuff I did find that my legs rubbed on it a bit from time to time but it wasn't really a problem. Getting stuff out is simple enough, with the hinged lid opening away from you to give you easy access. The cover extends down over the base to help keep water out. It's not claiming to be waterproof but it'll shrug off showers easily enough. There's reflective piping around the lid to give you a bit of extra visibility after dark.