I have:
A complete set of Arkel Grand Touring Panniers (2x GT18, 2x GT54 and an Tailrider.)
A set of older Trek Shopping Bag Panniers
A Rivendell Saddlesack Small
A Rivendell Saddlesack Medium (New)
A Rivendell Shopsack Large (New)
I also use a Timbuck2 Commuter messenger bag as my briefcase/ day bag. I've never been really happy with this bag other than it works.
Each of these sees duty as commuter gear during the year, depending on load/ errands. I think the GT-54s are a bit overkill for commuting, but having purchased them, I feel I'll never have to purchase another set of luge rear panniers ever again.
I am quite displeased with the on-bike performance of the Trek Shopping bag panniers. The attachment system is poor, I've had them fall off a couple times. I use them more often as tote bags rather than panniers.
The Shopsack fits the role that I wanted these panniers to play better, offers more support, better attachment (in a basket over the front wheel) and more flexibility.
The small Saddlesack is great for a day ride or spring/ fall commute when I'm not carrying a lot of stuff. I'm a "be prepared" kinda guy, so I probably carry more stuff than would be common. Extra windbreakers, a button down shirt, extra socks + food can be easily carried.
I bought the medium Saddlesack to become my go-to commuting bag. Time will tell if it fills that role well or not.