G1 viewfinders are, well, viewfinders. A G isn't an SLR, and yes, the viewfinder is somewhat darker than the actual scene, but that's how it is. As the owner of four G1s, and two 45mm lenses which I use regularly (I usually carry one G for B&W, another G for color negative), I long ago got used to their (few) limitations and learned to work around them. the 45mm isn't my preferred lens as I find the image too 'narrow' for my liking and prefer the 35mm or 28mm lenses, but all G lenses produce superb quality if used correctly. Overall I find the Gs have better than most RF viewfinders. My brief time with Leica Ms was in the 1980s when I was younger and had better eyesight. Ten years years ago I used a friend's Leica IIIf for a year and its tiny viewfinder all but drove me insane, but this said, there is no sensible comparison between Leica IIIf and G viewfinders. I cannot afford Leicas now so the Contax Gs are, for me, a fine option and in no way inferior. I use my G1s every weekend, and am happy with the viewfinders. As a fussy shooter, I like the results from my G lenses as what I see on my negative(s) is (almost) spot on with what I saw when I shot the image(s).
To me the only 'minus' about the G1 is the noise the camera makes when it focuses. I quickly got used to the viewfinders and I continue to be very happy with my results. Many others don't like the viewfinders. Each to their own...
One person's viewpoint, this, from almost 20 years of use.