I bike to and from work and school and my panniers always stay with me loaded with laptop and paperwork (and usually some tools and bungees). I use Arkel B-40s because I wanted a large single compartment - they come as a pair and I either have the right side bag locked to the bike (locked through a metal tab, not a strap, so they need an Allen key to remove it or have to cut the lock) or leave it at home. The left side bag is removed in seconds and gets carried with me everywhere.

I used to bike with backpacks and I would always leave the ride with my back completely soaked. The panniers have the added advantage of giving drivers a visual cue closer to the true width of your bike, lest they judge passing space using your rear wheel instead of your handlebars - that's why I keep my left bag on all the time and clip a blinky light to it as well.