I've had the Topeak MTX DX bag (no panniers)
& Explorer rack for ~3 years now.
I can fit shoes, jeans, shirt, & under clothes
in the main compartment with no problem. Fitting
a lunch in there too would probably be a problem.
In one outside pocket I keep my tools, tube, patch
kit, etc. The other contains the battery pack for my
rear light, spare cells for front lights, & the alternate
lenses for my sun glasses. I keep my cell phone,
wallet, & other misc. stuff in the top compartment.
This is a fair amount of weight up high, affecting
the center of gravity, but I'd rather deal with
that than the bulk of panniers hanging in the wind.
I've had no real issues with it, but it is something
to be aware of.