I've used two side-access backpacks.
I've had a Lowepro Primus AW (bought it, used it for one trip, sold it). The access opening on the side of the Primus was simply too small – I was using a gripped 7D, the grip was a non-starter but even with it removed it required a good tug to get the 7D + 17-55/2.8 out, and putting it back was such a struggle that it was easier to just take off the pack and lay it down.
I currently have a Lowepro DSLR Video Fastpack 250 AW. It's a decent design – the side access is also the main access, a vertical nylon strap limits the zipper flap from opening too far with side access, which can be unbuckled to allow the flap to open fully for complete access. So, basically side access works great for the body with the mounted lens. However, changing the lens is another matter. An opening wide enough to allow access to extra lenses is wide enough to let them tumble out.
One thing I like about the Flipside is that it not only allows access to the other lenses, it serves as a platform for a lens change.