I use a pannier rack and bags as well as a top/trunk bag. On my Seek 2 it's a SunLite PakRak rack and top/trunk/grocery bag. ON my Hardrock it's a Topeak rack (for disc brakes) and a "no name" pannier bag that has an attached top bag that can be unzipped and used as a backpack with mesh netting for wet clothes and to carry one's helmet. I thoroughly enjoy that setup, but it is in serious need of repairing.

Both pannier sets of bags are joined together and have a handle to carry them by. Really the only time that I see myself removing them is when I go shopping. Then they hang over the side of my shopping cart and I fill them as I'm shopping.

I do have an excellent backpack with frame and padded waist belt and sternum strap. It's the large military backpack I purchased and used when I was on Active Duty in the US Army. I do NOT see myself wearing that as I'm commuting.

Actually I can't see myself wearing anything larger then my 3l GI CamelBak.

As others have said, I would much rather have the weight on my bicycle rather then on my back. YMMV