I love my backpack. It's a Wingnut Adventure Pack. I don't even use my Ortlieb panniers. The reasoning is that my backpack has a lot of small pockets, perfect for organizing. When I get off my bike all I have to do is lock up the bike and walk away. No awkward pannier to lug around. The backpack just becomes a part of me and I hardly notice it until I need something out of it. My essentials are in there all the time so I never even have to think about what to bring. Carrying the usual stuff is not heavy at all, either. I can throw in an extra jacket in the main compartment when it's cold or food for longer rides. I don't like putting too much stuff in it because then it rests weird on my back, but I could. It also has a pouch for a hydration pack for the summer. I love it.

If I were to try something else, it would probably be Arkel's Trunk Bag. It looks like it has awesome design features and that makes all the difference.

The only reason I see for riding with panniers all the time is if you carry a crapload of heavy and or bulky stuff every single day, but I don't. Also, grocery shopping is easier with panniers.