My wife has been using one for commuting for the past week or so and said she is very happy with it... She used to carry a back pack but got tired of how sweaty it made her - and the backpack.
She carries a change of clothes, tools, and lunch in this bag every day with room to spare - it's got a deceptive amount of space inside for it's size and the price is unbeatable for the quality of the bag. ($30 CDN)
I picked one up because my panniers are monstrous (they're for motorcycle touring, but they where free) and I have a nice CCM trunk-and-pannier combo which is good when making a one-stop-grocery-haul or commuting with a lot of gear. Both sets are really impractical for multiple stops (no carry handles, need to be unloaded when mounting, and they're huge)... So I needed something smaller that would allow me to still use panniers if I needed them, so this seemed to fit the bill.
I also looked at the Topeak trunk with the zip-down panniers but they looked a little small, thin and flimsy... I figured groceries would wreck them fairly quickly and it would then be just a trunk bag with useless side-pockets.
This trunk bag attaches easily with four velcro straps (your rack needs to have side-rails) and has a convenient shoulder strap and a mesh helmet-holder on the top. I use the helmet holder for my lock when I'm riding so it's handily right-there on top when I stop and lock up.
The velcro straps keep it quite stable when loaded and unloaded, and the main pouch expands if you need more room.
It has two side pockets - I use one for bike-fix-tools and the other for human-fix-tools (ie. first aid kit).
Both side pockets have zippers have an extra mesh pocket on the outside... I use one mesh pocket to store the shoulder strap when not in use.
It also has a smaller rear pocket where I store my lights when riding in full daylight so I don't have to unclip them when I park, but can always have them with me if I'm out somewhere and it gets dark.
There is a rubber hanger for your blinky light, but none of mine (all are Torch brand) work well with it, they seem to end up pointing at the ground so I just mount them on the rack.
A nice touch with this bag is it has two velcro straps on the right side for holding your pump as well.
I was sceptical about it's carrying capacity until earlier today - I was able to fit a six-pack of tall cans and two bottles of wine in the trunk without even using the expansion capacity. It's insulated so the beer where even still cold when I got home. My wife has managed to fit a LOT more in it by using the expansion zipper to make the trunk bigger and using the helmet holder to strap down a grocery bag on top of it.
It's water-resistant but not water-proof, you might need to wrap your contents in plastic during poor weather, however no bag in this price range is going to be water-proof.
All-in-all this is a great trunk for it's price and if you're not carrying the kitchen sink on your commute I'd not hesitate recommending it.
This thread post has pictures of the bag expanded and loaded as well as closed and nearly empty, while attached to the bike: