I've said a lot about Rub N' Buff in my build thread, but I'll add my input here too.
One thing I hear a lot is that it seems silly and people liken it to "makeup for your engine". I think that's a silly way to look at it. I've spent less than a collective hour applying Rub N' Buff to my engine during my rebuild, and it is going to prevent oxidation and ensure that the engine looks good for quite some time. Most people don't think it's silly to wax your tank or fenders to protect them and keep them looking good. In my mind an engine should be no different. I've worked very hard to remove all of the aluminum oxide from my old girl, and I think with a single application per year over the winter when I can't be riding anyway, the oxidation will never return as long as I own this bike.
From what I've seen (I haven't used it), Pewter is a good color if you want to achieve the look of clean but used and road-worn aluminum. Silver leaf is a great color if you want your parts to look very close to freshly media-blasted cast aluminum. Very close, but not perfect. There is a slight difference in the tone. I've attached a few photos for comparison. My take on it is that the RNB has a slightly cooler hue to it, and the natural aluminum is slightly warmer. If you've ever seen a chrome wheel next to a highly polished aluminum wheel, I feel this is very similar. The chrome wheel will be cooler in color with more of a blue tint, and the aluminum will be warmer. It is a subtle difference, and I think that anyone would be VERY hard pressed to determine if a clean engine did or did not have RNB applied without a second bike to compare it to.
Engine block has RNB, valve cover does not:
Engine block and covers have RNB, cylinder, head, and valve covers do not (yet):
Another thing I hear is that it looks too fake. I think a lot of this comes down to application. I've learned a lot applying the stuff over the course of my project. First, DON'T USE TOO MUCH!!! Less is better. If you put a pea-sized dab on your finger, you're overdoing it by a long shot. You need a tiny fraction of that amount. If you put too much on, it will completely fill all of the casting crevices in the aluminum. This makes the surface appear smoother, and I think this is what many people see that makes them think RNB looks too fake. If you apply very little at a time, lightly, it will just fill the high spots and leave all of those little dimples, and will look much, much more natural, as you can see here (RNB on all the cast aluminum in this photo):
You can also see what I'm talking about in the photo above with the engine block and the valve cover. See how you can still see all of those little dimples in the aluminum in the engine block? That's what you want to go for. If you fill them in, and you can, then the whole surface looks too much like plastic.
The other thing is buffing. The less you buff after applying RNB, the more dull of a finish you get. The more you buff, the more of a metallic sheen you get. In my opinion you can both underdo and overdo it. There's a happy medium when buffing that gets just the right sheen and truly looks like clean aluminum. It's not rocket science, buff a little and keep going until you get the look you like. This all sounds like a pain, but it can be done very quicky with very little effort.
To address the question about black RNB on the frame, it doesn't work nearly as well as RNB does on the cast aluminum bits. In my experience, RNB works far better with porous materials. The smoother the surface is, the more poorly it will work. I just bought some black RNB just to try it out on a few areas of the bike.
Here's a nasty scratch that I managed to put in the swingarm because I didn't remember to cover it when remounting the engine
Sorry the "after" shot was out of focus, but you can see that the RNB did get rid of the color difference, but does nothing to fill in the scratch. It looks better for sure, but not perfect. It will have to do in this case since I'm not going to have the swingarm powder coated again. It's certainly better than before.
Here's my clutch lever perch before RNB:
Shinier, but not much better looking (in my opinion). I was hoping this might do the trick, but I now know I will have to paint or powder coat the perches.
This is how much I used to do both jobs above, and it was too much:
So, the stuff definitely has its place. I think it's a great solution and it can make very poor looking parts look near brand new very quickly and very inexpensively. If someone was the really lazy type, they could apply RNB straight to a dirty and oxidized airhead engine and make it look a million times better in about an hour (with the majority of that time taken up by those darn cylinder cooling fins) for under $5. Not a bad investment at all, if aesthetics matter to you.