I bought this bag for a three-months trip to South America. At the beginning I was considering to get something bigger, but 3months with a heavy load on your back can be very difficult. This seemed to be the best compromise in terms of size, internal storage and space, pockets and - very important - shape of the back side.
45 plus 8 will allow you to put all the stuff you need (keep in mind that the more space you have available, the more you will fill it..), and the bivi-divider is an excellent feature to allow you to keep dirty or wet clothes at the bottom, completely separate from the main compartment. What else? Rain cover, biofit back, two mesh pockets on the side, access to the main compartment from three different parts (bottom, top and middle) plus other three extra pockets (two on the side, one on top), water-system compatible, fully adjustable and above all VERY strong and durable material. I do like this bag, it's going to be a good travelmate!