I just finished mine, looks good. Went with red opposed to black even though the truck is Onyx Black. There's enough red on the truck as accents already that I didn't want to have to go and make everything black as it would have bothered me.
Just to give some bit of experience from my install:
Took me 60-90 min actually, which seems to be considerably longer than most here but still not bad.
I spent a lot of time trying to route the wires and harness as best as possible to avoid seeing it through the grill. Probably spent 20-30 min on that alone. I did end up routing it similar to pictures from the 1st page incidentally. Was difficult to squeeze through there but got it done.
Even after disengaging the clip on the headlight hardness, it was still very difficult to get it out. Probably another 15-20 min here as well. I don't know if my hands are bigger or something but it's just difficult to gain any sort of leverage. I'm sure it would have been easier if you removed the headlight assembly per the instructions but even with the wasted time, I was happy not to have to remove the headlight assembly.
Taking the black cover off between/over the grill and radiator is essential to a successful install. Removing the grill might have made things a bit easier, and I'm not sure it is very difficult to remove. I saw 3-4 bolts but it looked fairly simple. In hindsight, I probably should have removed it but it isn't required.
Saving the existing emblem is only possible if you remove the grill in my humble opinion. Disengaging all the clips on the existing emblem seems very difficult to do without damaging them if the grill is still installed and the red inlays are still in. Prying the red out of the chrome surround makes it easy but you will destroy the existing emblem this way. No way around it. I actually broke two small flat screwdrivers trying to pry out the red. Tips of them snapped clean off. They were cheaper screwdrivers but still, it's a tight fit. Should have just went at it with a bigger screwdriver not caring about damaging them because they cannot be saved.
Overall impression:
They are not terribly noticeable in the daylight, which I like. I haven't seen them in the dark yet but it's a good amount of light to not be obnoxious but still give it some pop.
Another user mentioned that the chrome surround is lost and that it looks better. I agree. The grill on my AT4 doesn't have much chrome really except the emblem. It's a darker more subdued grey for the most part in other areas. This makes the emblem fit better on the front end for sure, visually. Unless you really like the chrome. I know many other Sierras go full chrome in a lot of places (like the Denali) so if you like that look, you might think it's a small step back. If you have an AT4 though, it's perfect, as they eliminated a lot of the chrome on that trim.