I wonder about the effect it has on performance. When I play Fallout NV, I use an ENB, and I used to see a pretty big performance drop until I upgraded my computer. Now it's not as bad as it used to be, but the performance hit is still there.
I also like the look of darker nights and the glow of the coronas, but that can be achieved by adjusting settings in-game. As far as the daytime settings, I think it looks a bit unrealistic in terms of just how much the colors are saturated. The colors don't really blend and the result is it looks like somebody turned the contrast up a bit too high.