I only had two things I wanted on my bar, a bike GPS when I get one, and a small flashlight, so ended up getting the Topeak BarXtender so I could my light above my Topeak Tourguide DX bag.
I did have some issues. The Topeak Tourguide DX bag had a extend bar on the bracket but that stupid thing wasn't high enough for a light to clear the bag, poor design on Topeaks part, or it was done on purpose so you would have to buy the BarExtend and have a place to mount it to, so that is why I got the BarXtend, I mounted that to the bag extend bar to the bracket bar but ran into a problem. The bracket extend bar was not real secure, and once a light was mounted to the BarXtend the feet on the bracket would rock up and down, so to fix that problem I put two zip ties around the feet and pulled them as tight as could to prevent the feet from rocking, and that solved that problem. Topeak did not design that bag bracket real well, but the bag itself is fantastic, very roomy bag.
EDIT: I forgot to mention something about the Topeak BarXtend when I first posted this stuff. It isn't strong enough to hold my Phillips SafeRide light without seriously bouncing around, but that light is heavy and not balanced well, but something to consider if you put a GPS plus a large light on it. I can put my small Lezyne Micro 400XL on it, plus a VDO Z series bike computer without issues after I braced it better, so a GPS with that Lezyne or similar light would be fine, just not a tank of a light like the Phillips.
Anyway, now my light is mounted plenty high enough so the light beam clears the bag and it doesn't bob up and down as I ride. I carry spare zip ties with me when I go away on the bike, in case something happens to my zip ties on that bracket I can replace it, but zip ties are handy for other stuff too.