Speaking from personal experience, cheap rucksacks/daysacks are not worth the money. Not if your wearing it for any lenth of time. Best place to look for something decent is any top end outdoors/climbing shop. Look for the following: ripstop lightweight construction, shoulder straps that are a comfortable, generious fit with long straps. Cheap bags tend to have very short, poorly fitting straps. Look also for good quality zips and deep bottle sized mesh pockets that are easy to access when wearing the bag. Good quality ain't cheap but the degree of comfort it gets you is well worth the money. They are also a damnsight more durable than the cheaper stuff so longterm you might find "expensive" cheaper in the long term. Wear your basic load in a cheap bag then borrow a good bag for the day. Just to see the difference for youself.