Here's what I found after a few months of riding with the Kriega bags.
My normal setup was the single 20L bag attached to the Triumph luggage rack right behind my rider's seat. I removed the pannier seat. I got two 10L add on bags initially, but have never used them.
Small and unobtrusive
Knowing I could add one of my 10L any time I wanted more space
The 20L fit my satchel, laptop, extra cloth and googles but just barely
Lots of fiddling around with straps and clips every time you add or remove stuff
Top exterior zippered compartment got my Triumph manual a wee bit wet in a rainstorm. Interior compartment 100% dry.
I didn't want to add a 10L because then it would be just another set of straps and clips to have to mess with
If I would do it again, I would order a 30L Kriega instead of the 20.
I actually just ordered the small bag from www.bikersfriend.com
and I'm looking forward to seeing if it can replace or augment my Kriega system.