For the backpack detractors, there are some advantages to using rucksacks:
- no need to fit a pannier rack, or have a bike that has the appropriate eyelets
- a pannier has a fair amount of drag
- the obvious solution if you have more than one bike
- easier to carry around off the bike; a pannier is a rather awkward thing when you're on foot
- not bike specific, so just as useful for things such as hill walking or just walking to the shops
I used a rucksack for years commuting, usually going home the long way (18+ miles), often carrying a laptop. One thing to note is it acts as an extra layer - not so great in summer but quite welcome in colder weather. The one thing I'd advise is getting one with a mesh backsystem which allows air to circulate between the body of the pack and your back, reducing sweaty back syndrome.
I use a Lowe Alpine AirZone 35. It's not waterproof but does come with a rain cover. You'll find similar from other manufacturers such as Deuter or Berghaus.