To start with, this item arrived very promptly - I ordered it Sunday evening, and it was on my doorstep Tuesday morning, which I thought was brilliant for free postage.
The bag itself is very roomy, and comfortably holds a spare inner tube, an assortment of tools, a mini-pump, plus keys, phone and cash. It appears well-made of tough materials, and the zips and clips look sturdy. Fitting the bag under the saddle is easy - one strap goes round the seat tube and fastens with velcro, and two other straps fit through the seat rails and fasten with clips, and these straps can be pulled up tight so the bag doesn't bounce about.
There are two elastic loops under the bag, in case your pump is too long to go inside, there is a loop on the rear-ward facing part of the bag which takes an LED rear lamp, the lid of the bag fastens with a zip and there is a net pouch inside the lid so you can easily find small items such as keys, and a second zip allows an extension to the length of the bag for even more storage space if needed.
I am delighted with this bag, and highly recommend it, and the seller.