I have a brown and blue purse sized one; I use it in the summer, when I don't need to be "professional" looking. At one point, I tried riding to work with a full sized messenger bag; in fact, I have a video of my attempt. It was just too much weight to make me unbalanced, since my coordination is questionable at best! I ended up using that bag as a gym bag for many years.
The purse sized bag could be used for a trip on my road bike, where I don't have space for any bags. But, it doesn't have the stabilizer strap.