Basically, it is a very hard coating, metalic in nature, so it's not paint or ceramic~not Cerakote and is typically a beige to almost gold color (unless you treat/paint it differently) that is very, very hard and somewhat like teflon coated metal (but much tougher) it moves against itself and other metals with very little resistance. It means the firearm typically cycles a bit better, needs cleaning much less and when it does need cleaning, it cleans much easier.
Some sites say it needs no lubrication, but I don't believe that. I think a very little bit of lubrication is good, but as always, too much lubrication tends to attract dirt and 'gunk up' a gun--so again--a little dab'l do ya...
I don't recall the exact Rockwell scale of hardness N.B. typically has, but it's much harder than the protective Melonite of Tenifer coatings found on many firearms. I have not seen information on whether this great hardness means it can be brittle if it takes a strike (like ceramic knives) but they're using N.B. in more and more assault weapons, including in the military, so I am inclined to think it's not a problem, unless I hear otherwise as time goes by.
To me, time is the ultimate test and for example is why the 1911 sidearm deserves it's excellent reputation and also why Glock, (until they went and screwed up Generation Four), had a sterling reputation. Nickel Boron's been around a while too--especially in tough industrial applications--I think for about 50 years. In ten or fifteen years, I am optimistic that the S&W M&P series will have earned a similar reputation--so far, so good!
I've read that you can polish the typically matte finish to a golden chrome like gloss, but that's not my style, especially on parts that are exposed to sun and other bright light, potentially creating glare. I don't have any such issues with the typical, standard N.B. coating. Unlike some of my outdoor guns, I don't see any need to treat the upper surface to reduce glare and enhance target aquistion time.
I've been happy with N.B. on slides and barrels and when it's not a high priced after the sale option, I'd strongly consider it again.
I'm not one to jump on th bandwagon for the newest thing and as said, this has been around a while, but just seems to be getting to small arms more recently. S&W lists a N.B. coated M&P on their official website, but oddly indicates that the dealer determines the price, so beware the 'gouge factor'--I don't think it should cost more than $75 or so above the standard Melonite coating, if they do the slide and barrel, as is typical.
I bought a new N.B coated Glock 27, (gen 3) with Trijicon night sights, 3 extra magazines and all the standard Glock 'stuff' and some Corbon ammo, for an even $600 at a gun show. An outfit called WMS did the N.B rather well. EXO is a well known name for N.B. as well, but there are others as well. There are some decent Youtube videos also on nickel Boron coated weapons.
There are a number of links on Google when you type in 'Nickel Boron coatings explained' that will tell you more: