Just to make this clear; the cover is advertised as waterproof, which it is, but it will not make the contents of your rucksack waterproof. This might sound a bit pedantic, but I think it is worth pointing it out, just in case you are a customer shopping around for something that can be immersed in water. This is a rain cover and a very good one at that.
I bought this for a Lowepro fastpack 350 and it covers it quite nicely and adds that extra level of protection should I get caught out in a downpour. It is a bit like a giant elasticated shower cap that you pop your rucksack into and it is quick and easy to put on and remove. If you have vented back panels this may impede the airflow as the elasticated band hooks around to the back, but I cannot say for sure.
By its very nature the cover will interfere with pockets and access to the rucksack, in particular fast access panels. However, it is unlikely that you will want to get your camera out when it is pouring down. If you do need access, then you just pull the cover to one side.
Because it is so quick to put on you don't have to keep the rucksack permanently covered. If the rain stops you can simply pull the cover off, give it a good few shakes and store it back in the supplied pouch - remember to dry it out thoroughly before long term storage.
I have been in rain on a few occasions with this and the rain just beads off the pack. The water did drip down onto the backs of my legs, but I can easily dry my legs, my camera is a different story. Most rucksacks have a limited rainproof capacity, but this is a lightweight pack that will give you added protection when caught out in a deluge, worth having stuffed into a side pocket.