I think they're great, I use mine for multi day trips and it holds everything I need with the exception of my sleeping bag, which I put under the handlebars.
Mine wasn't very stable until I threaded an extra strap through it, a five second job which meant I could jump and play on the bike even when I was carrying 4 days camping gear.
My shoulders do end up a bit tender though, I've been thinking about padding the shoulder straps a bit more to prevent this. Also, the bags are built lightweight, and the zips have burst open once or twice when I've overstuffed. So long as you're careful not to stuff them to bursting (hence the sleeping bag under the handlebars) you should be fine.