I'm a huge fan of things that serve two or more functions, like jeeps that are also boats. And so, before my recent trip to Europe, I shopped for a small backpack for hiking and a small black 'man' bag for evening. My requirements for each bag: they must be big enough to accommodate my iPad 2, have an external water bottle pocket, and at least one other external pocket for easy access to pens, pads, keys, and such.
I ended up buying only one bag, the Victorinox Flex Pack ($63 at Amazon). With its smart design, the bag can serve as a compact backpack. Then, by reconfiguring its two straps into one, you can wear it across the shoulder bag. The strap is said to be slash-proof because it's reinforced with wire.
Downside: There's no cushioning to protect your iPad. So if you use this as your iPad bag, be sure the tablet is protected with a sturdy case. And that's about it for drawbacks, as far as I'm concerned.
One other thing: I bought a small (13 oz.) filtered Bobble water bottle for the trip. In the morning, I'd fill it with tap water in my hotel so I'd have water with me throughout the day. And the small bottle fits perfectly in the Flex Pack's outside water bottle pocket.