I installed the drivers for my 290x after uninstalling and wiping the old beta ones. So far it's all good. I've tried to downsample 1440p to my 1080p monitor and the results were generally pretty good.
The biggest difference is in GTA IV which has no native AA. 1440p downsampled + the SMAA injector gets rid of the jaggies, though performance does take a bit of a hit. I've had good results with Crysis and Alien: Isolation as well.
I also tested this in Euro Truck Simulator 2 and Arma 3 to see how it fares compared to their native downsampling options present in their graphics menu.
In ETS 2 I had 1080p + MLAA + 300% scaling. I've tried 1440p downsampled + MLAA + 200% scaling and the image quality was slightly better and the performance similar.
For Arma 3 1440p downsampled was negligibly faster ( +0.5 FPS at around 50 FPS) compared to the existing 133% sampling option and it blurred the interface while looking roughly the same.
One game where downsampling did not produce any benefit was Civilization 5. It shrunk and blurred the interface and I found the native 4x/8x MSAA options to be better overall.
I suppose whether downsampling produces benefits depends a lot on the nature of the game and the already present AA options. It is superior to no AA and shader based AA. Interfaces and HUD's may be only minimally affected (GTA 4, Crysis, Alien: Isolation) or may become almost unusable (Civilization 5). When the game does have native downsampling (ETS 2, Arma 3) then it may or may not be worth it.
In any case I didn't do a lot of testing and will have to see how this feature impacts performance and image quality in the long run.
PS: I've noticed that when downsampling the refresh rate goes from 120 hz to 100 hz. Is there a way to change it back?