I love my computer and I do not want it to break if I go away for the weekend because I have dropped it or because of a little light rain, this is the reason I bought this case.
This was around £20 in Currys about two years ago.
it accommodate laptops with a screen size of up to around 18" comfortably and possibly even 19" but this could be a bit of a squeeze. The laptop slides in from the top into a special padded compartment which is at the backmost part of the case so that it essentially buts up to your back through a foam wall. The advantage of this positioning is that there is less chance of it breaking if something happened to hit the bag whilst it is on your back when you are walking. There is great protection on the sides too as the special laptop compartment features a wall of air within flexible plastic tubing along the spines. These pockets of air means that any side impact will have little to no affect on the laptop whatsoever as the air cushions it superbly. Being air, this protection weighs nothing and therefore portability is enhanced greatly!
The case is the standard material used on most backpacks (I am not a textile person so I am sorry I do not know that the material is called). The disadvantage of this is that it isn't waterproof. The case can withstand some light rain as there is further protection inside but it is not watertight. When I put my laptop in the case, I liek to wrap my laptop in a rubble sack first to make it water proof - this way even if there is a very heavy shower, my laptop is perfectly safe. there is some slight protection by the slightly rubbery/plasticy decorative lining on the front of the bag but this is still minimum protection.
The bag is very comfortable to wear. Partly due to being lightweight but also due to the large amount of padding put into the shoulder straps which cushions the weight of the laptops on your torso. I can wear the bag all day without much back trouble.
A great aspect of this bag is how normal it looks. By this is mean that unless you actually examine the bag carefully, you will not really notice that it is a laptop bag. When walking through a busy city you do not want to be wearing something which essentially is shouting to every near you "Hey, I've got £800s worth of computing equipment with me!". This bag is very inconspicuous.
A downside to the backpack styling is that you cannot see if anybody is tampering with the bag behind you as you walk. In a crowded city is would be incredibly easy fro somebody to undo the zipper and slide the laptop out. You would most likely notice the change in weight on your shoulders but by then they could be too far away for you to catch them. Luckily, you can attach a luggage lock to the zipper in this case so that nobody but you can access the laptop. With enough time and force anybody could usually break the lock but I am pretty sure you would notice somebody with pliers attacking your bag. The downside to having a lock is it makes it more obvious what you have with you