If you are looking for an ultralight daysack these are great.
I bought one a few weeks ago and I’ve used it almost constantly for day walks. At 2.4 oz /64 g it is extremely light and packs down to a tiny packet you can clip on a keyring. It holds around 20 litres which is enough for a set of waterproofs, a sandwich or two, two litres of water, a fleece jacket, camera and first aid kit plus other bits and bobs.
The pack is very durable and the stitching is holding up well. There are no claims by Sea to Summit that the pack is waterproof but my experience has shown it to be highly water-resistant. Zips and strap adjusters are very robust.
Because the back is unpadded I had an issue with waterbottles etc digging in so I’d suggest you do the same as me. Cut a piece of thin foam rubber to act as padding. As a bonus you’ve got a dry seat to sit on while you’re having your food!
As well as a daysack, other uses could be for carrying stuff to work and back, as an emergency shopping bag or for use when base-camping and you don’t want to lug your full-sized bergen around all day.
At around £16 they are a bit pricey but very useful so recommended. Manufacturer’s link here http://www.seatosummit.com/products/display/86