I have a 2010 Kia Ceed SW which I've owned from new. It started smoking badly at startup when only six months old. It turns out this is a known problem and found that owners have had repeated trips to the dealer while they change glow plus etc and still not properly fixed the issue. Then someone on the Kia forum recommended using Millers EcoMax, so I tried this first. To be honest, I wasn't expecting it to make any difference as the smoking was quite bad and as this was a new car, I wasn't expecting there to be any "cleaning" of the injectors needed. I used double strength first, the normal, it cured the problem. Now a year on I'm just about to purchase my second bottle. If you use supermarket fuel, then this stuff is needed every now and then, and it really does make a difference.
I don't use the Millers additive all the time. I haven't used it for about 4 months and I noticed the smoking was starting to appear. I then used it and never seen any smoke since. I try to buy Shell or BP diesel when I can, but often just buy the cheapest (supermarkets) and therefore I use a shot of Millers at the same time.
I would have never believed this stuff would make a noticeable difference, especially on a new car, but it does. I suppose it depends on the quality of the fuel you use, but if you buy cheap fuel, then you may need to use this too. There's a lot on the internet about the different additives the fuel companies use when delivering to the different forecourts, so it does make sense why I needed this.