Let's start with some technical specs data. The flash will do manual or TTL. It supports high speed sync. I am hoping at some point in the near future to test it with a light meter against a Nikon SB-700 (don't have a 910 available to test against), but in my experience in the past it sits just under a 900/910 and a little higher than a 700 in light delivery. I have learned to not trust guide numbers, because they don't seem to compare well. The 685 has radio receivers for both the RF-603 and the YN622 systems. So you can mix this light in with the YN560 IV and either a RF-603 or YN560-TX in manual mode and have full control of the light in any of six groups just like any other 560. Or you can put it with other YN568 with a YN622 receiver. I have even seen YouTube videos of people mixing in Nikon SB-900 lights with the YN622 receiver and having full TTL with multiple groups. If you have a YN622n transceiver then you can put one of your 685s on top of the YN622n on the camera and use the 685 display to control the settings of the flash units in all the different groups. Although, unless you want to shoot a flash from the camera too, just get the YN622-tx for on top of the camera.
So this light has a lot of features! With that level of features the light is somewhat complex to run. The manual is not horrid, but it is definitely sparse for information on setups. They could do a little more in that area, but it might raise the price of the unit. So take some time to get used to how it works before trying to use it in a paid shoot or high pressure shoot. Once you figure it out the functions do make a lot of sense for the most part. And I am not sure how they could have really made it easier. I am going to start making how to videos and will start to help walk people through using these.
Let's talk about high speed sync. This is a really cool feature that you don't get in monolights unless you are paying some serious money. What high speed sync lets you do is to shoot above the flash sync speed of the camera. I was able to shoot all the way to 1/8000th of a second on my D7100 without a problem. The Yongnuo Canon version supports the same thing. Now, one cautionary note on high speed sync on the Nikon. You need to have at least a D7100/7200 level camera or higher for high speed sync to work. That feature will not be available on a D3000 or D5000 series of cameras. To enable it you need to go into the menus on the camera and set the flash sync speed to Auto FP. This is the feature missing in the entry level cameras. Sorry, just the way life works sometimes.
When you are shooting with high speed sync one thing you need to realize is that as you increase your shutter speed past flash sync speed (1/200th or 1/250th) then you will get less light from the flash. You are not really losing power from the flash. Some people say that, but it is not actually true. What happens is that the flash will pulse the output so that it flashes repeatedly over the duration of a 1/200th of a second flash. Think of it as a 1/200th of a second constant light. So as you increase your shutter speed you are actually sampling a smaller portion of the total light output from the flash. So you will have to adjust aperture or ISO accordingly. The nice thing with high speed sync is that now you can knock down ambient light at wider apertures and get a good exposure with flash. The high speed sync will even work when the flash is in manual mode. So you are not forced to shoot in TTL to use HSS.
So now we get to the bottom line... the cost of all this awesomeness. The Nikon SB-700 runs around $325, and the SB-910 is over $600. The YN685 will do everything either of these will do, and I got it from Yongnuousa.net for $114!! Yes you read that right, for just over $100. So you can get three of these for about the price of a single SB-700, or five of these for less than a single SB-910! And the radio receiver is built in! So you don't need to get separate radio receivers for each light. If you are going to equip for even a simple three light setup consider you can now do it with Yongnuo for the cost of a single SB-700. If you shoot weddings, commercial, or other high stakes time critical shoots you can easily afford to have multiple backup flash units in the bag just in case. I will say that I have yet to have a flash failure of my YN560 units other than ones I dropped onto pavement so far. I have 6 of the 560. There is always the possibility of failure. But that is also true of Nikon and Canon flash units. But these lights are incredibly reliable.
One other feature I forgot to mention that is really great for people that do event photography like wedding receptions and proms and the like. The YN685 has an auto focus assist beam feature that is out of this world awesome. What it does is to project a red laser grid briefly out of the front that the camera can use to focus. Then as soon as the camera locks focus the grid turns off. So it is on for just a very short time. This way you can shoot in pitch black with flash and still get your camera to auto focus. I got my light a week after I shot a prom and so wish I had that feature available. I have shot around the house at night with all the lights off and the auto focus assist works perfectly!