I used a Camelback Trans Alps for a year. It worked well, but it was a little difficult getting my clothes in and out. The collection of pockets worked very well. I noticed I was distinctively faster using the backpack rather than panniers. The biggest problem sweat infiltrating the fabric. The pack is rank . My wife has banned the backpack from the house. That makes it much more difficult to loadup for the commute.

I have switched back to panniers. I currently use Ortlieb Sport Packers and a trunk. I preload the panniers with clothes, then just take one one. My lunch and other bits go in the trunk. I'm quite happy with this solution.