This is my third Ospey pack and I still use them all for different purposes. I have had Gregory, Dana Design (before they went downhill), Mountain Hardwear, Camelbak & Arcteryx (they should stick to clothing). I will keep buying Osprey unless they change something. I love their hydration system and with one small and one medium pocket, the Viper is on the small side but super light and comfortable.
The one knock against them is that everyon seemes to agre is their packs seem to hold less than other brands of similar size. Maybe it's how they measure them. The Viper 7 is supposed to have 400cu inches. That's 4x10x10 I don't see that much usable space in this pack. I put in a couple liters of water, a wallet, phone, keys and a lightweight fleece pullover and it's packed tight (not counting the outside bungee). If in doubt, get the larger size. Osprey's are typically amongst the lightest so no downside.