Chris King is one of the companies that started the whole boutique parts thing. While a lot of the companies that came on the scene with no more compelling product attributes than purple anodizing have gone away, King has become the standard in whatever category they're in. Hubs are certainly among their better known products.
So first, let's do the numbers...
From a wheel builder's standpoint, the first thing about CK R45 hubs is just how good the front hub is. I don't think a front wheel can be made any better than one with an R45, all else being equal. The flanges are huge and far apart, and the bearing spread is outstanding. All of this contributes to an absolutely rock solid front wheel. Front wheels are the step-child of the decision making process, and they're symmetrical and inherently stable too, but it's worth noting.
Another notable and admirable facet of the King philosophy is that they make hubs that weigh what they need to in order to do what they want them to do. They state as much in a very straightforward way. They aren't heavy by any means, and I will put it simply that the amount of extra weight in a Chris King hub versus any other will make no difference to any rider under any use, ever. That said, they ain't heavy at all.
The rear hub uses a unique drive mechanism called a ring drive. Instead of pawls and springs like the other hubs we use, the R45's cassette body engages with a spring that pushes the drive clutch out when you're pedaling, and gets pushed in when you're coasting. The "45" in R45 refers to points of engagement (as opposed to 72 in their mountain bike hubs), so engagment is quick. Again, not that that's a huge thing in road hubs. Engagement is solid and reliable.
Both front and rear have bearing preload adjustment mechanisms which are easy to use. The instructions that come with the hubs are clear and easy to follow. King makes their own bearings, so you can't just get them anywhere and they do cost money, but they are extraordinarily good bearings and it's not uncommon for them to never need to be replaced. The hubs (but not the bearings) have a 5 year warranty, which is great.
Breaking the hubs down for simple service is extremely easy, requiring nothing more than hex wrenches. This allows you to do a thorough cleaning and re-lube of the axle and ring drive. Taking the hub completely apart requires a unique special, but this is really only necessary if you manage to completely and utterly mangle the hub somehow. The only reason we've ever had customers need this is to change to a new hub shell for different drilling. So don't be put off by that one, if you ask us.
This will be either good news or bad news depending on your preferences, but despite persistent rumors to the contrary, R45s are just not very loud. In fact I'd say they're about equal with WI T11s for being the quietest road hubs we use. The CK mountain bike hubs and their Classic Road hubs were/are louder and provide that reputation, but R45s are quiet.
Until Industry Nine and others came along with some pretty magnificent colors of their own, CK was the undisputed color champ. Navy R45s are still, to me, the most elegant looking hubs on the planet, and their iconic mango color is prized by, well, just about everyone. One thing to note on CK anodization, however, is that it does scratch somewhat easily. I guess the process of getting that thin candy-like coating leaves it a bit more vulnerable.
The elephant in the room on Chris Kings is, of course, their cost. They are very expensive. Does their value bear out the cost differential? Objectively, I can't say that they do anything so much better than our other hubs. Is a set of HED Belgium+ rims with R45s and CX Rays that much better than any other build anyone does? No, but they certainly are unbelievably nice, can be used for anything from touring to world class racing, will last nigh on forever with little input required, and convey a confidence and competence that's easily worth if for many people. It's so easy in cycling to find products that ask for extra money only to present you with deficiencies and limitations (many of you can guess what's top of mind right now). Kings ask for extra money, but if you can find a deficiency or limitation that comes along with it, I'd be interested to hear.