Out of all the rangefinders I've used I'd put it somewhere in the middle. It is not nearly as dim as say an Argus C3 but also not quite as good as my brand new Bessa R3A.
I have Leica M6 and two Yash 35, both Yashica's are bit dimmer than Leica, but I never had problem with focus even in pub shooting at 1000 ISO wide open
I carry a GS as my "truck camera" and basically if you can shoot without a flash the range finder is just fine. If you are trying to shoot inside, I was taking pictures at a graduation recently and it was a bit hard to see but I've never had a problem. The GS is my favorite carry camera, you really can't go wrong with one at the price you can find them for, I paid $2 plus $7 for one that looks like it just came out of the box.
I have a GT and the rf/vf is brighter than a FED and Zorki, dimmer than a Kiev and about the same as my recently cleaned Canonet. I'm hoping to have a Leica or Zeiss Ikon to compare it to very soon. =D