While the cube is strightly speaking not a bag (although I see you can buy a shoulder strap for it :)), I've been using one for liquids while travelling for quite a while now and thought it deserved a review.
The quality is as usual from Tom Bihn really good - it's been with me on numerous trips completely filled and I've experienced no problems with the zipper (which I've seen on competing products) or anything else. It cleans easily in case you get toothpaste on it and still looks pretty much as new.
For air travel it works really well - I put in on a key strap clipped to the inside of my Tristar or Western Flyer and when I need to go through airport security (mainly within Europe), I just unzip the bag and leave the organizer cube hanging on the key strap next to the bag.
99% of the times this is not questioned by airport security. In fact today while going through security in Frankfurt the security officer called her colleagues over and pointed to the organizer cube and repeatedly said "Sehr praktisch" (for those of you who speaks even worse German than me: Very practical). A few times I've been asked to remove it from the keychain and put it in a bin which takes seconds to do.
The only problem I've ever had was in Madrid where the security officer told me to use a plastic bag the next time but let me through with the liquids still in the bag.
Great product - can definitely recommend it.