I have one, the Exertec saddlebag. It's not bad - the flashing mode is a bit slow, but steady is ok (if not very bright compared to my Mars 3.0). Room for a boxed 700x20-28C innertube, repair kit and a spanner or two. (You modern types would probably fit a multi-tool in there, but the only Allen bolt on my bike is on the handlebar stem, everything else is regular nuts and bolts). Takes 2xAA, and the box for these does take up a fair amount of room in the bag.
The bag also expands (you unzip the lower, gussetted, portion to do this) but I have mine zipped so that it doesn't obscure the Mars 3.0 rear light mounted on the seat tube.