Just over a year ago, I wrote a quick review about the Sea to Summit Ultra Sil 20l daysack. You can find it here https://reidivinsmedia.wordpress.com/2011/07/10/sea-to-summit-ultra-sil-day-pack/
I’ve used the pack just about everyday. It carries shopping, documents, notebook computer, books, kit for a daywalk in the country and, once, even enough kit for a night out of minimalist camping. I’ve found it is pretty much waterproof and I often use it as a drybag within a larger rucksack to keep kit dry. Once camped, I can use the Sea to Summit for short walks or carrying supplies back to camp.
After over a year of use, the pack has held up extremely well. Some very,very minor surface abrasion but it is perfectly sound. So are the stitching and the shoulder straps. A minor niggle is that, under full load, the adjusters for the shoulder straps do slip a little but this can be solved by wrapping the free and of the strap round the adjusting buckle. You do have to rememeber that there is a limit to the weight it can handle but I’ve never had a problem carrying loads of 10-15lbs. I reckon it would be good for 20lbs but not a lot more.
As silnylon is thin I would suggest using a piece of foam rubber as a back cushion to prevent lumpy objects poking you in the back.
Minor niggles withstanding, the Sea to Summit daysack is the most useful rucksack I’ve ever owned. Tough, waterproof, very light, packs up to a very small size when not in use, reliable and incredibly useful. Highly recommended to anyone wanting a daypack.