I have the Knog Frank Dog. They are convertible bags that work well as both, and don't look like bike bags when off the bike. In fact, when mine was new, I was getting a lot of comments on how nice the bag was. They were always amazed when I showed them the bike attachment. They are limited in size, so not really for more than commuting or maybe a trip to the beach. The other downside is that it is a 2 point attachment to the rack and that's it. I always bungee it so it doesn't bounce around.

I was surprised they didn't catch on more as they are a lot nicer than the bike specific bags like Ortlieb.