I bought the Topeak rack and DXP bag with Panniers also, used a backpack and seat post rack for years, and would never want to
change back. I can not commute to work,(work out of my house), but rather commute mostly for shopping.
I have a choice of 3 towns to ride to, closest, 12 mile round trip, very small town, 2nd 22 mile round trip, a few
more stores to choose from, 3rd 35 mile round trip, everything you could want there. It is a little cumbersome
once you have the panniers loaded, so I try to work it out that I do everything I want to like eating out etc...
with the panniers folded up, then when leaving town do the big stop and load up...