I should give a little bit of background on how I paint, as Beardy Hammer would suggest, it goes way back to the 80's, so in that time there have been a lot of GW paint products come and go, in fact I still use many of my old paints.
My first GW paints were the Citadel Colour Space Marine Paint set from 1989, then I went on to get all the sets including the fledgling Inks set back in the day.
The paints were very different and quality sporadic (the Shining Gold was still the best gold GW ever produced mind you!) with some pretty simple shades. I didn't much like the inks, so tended to mix the paints and make my own washes (the Terracota colour in the Space Marine set was perfect for skin) and I've pretty much been doing that since, changing it slightly to using a leaching and technique over the past 8 years to get an aged look etc.
Suffice to say I wasn't into inks, apart from the black to thin down the black for black lining (does anyone black line anymore?).
So I was very very skeptical about Nuln Oil until i saw it in the Imperial Knight video and decided to give it a go. Well butter me up and call me toast...
Nuln Oil is actually great to use and straight out of the pot too.
Citadel Colour got it right on the money here, so much so that I have to restrain myself from not using it for all my shading as I tend to go very dark with my shading and leach most of it off surfaces so I can blend it back to the base then highlight.
As an example, for red, I base quite dark, then use a dark brown and base red for the shading, watered down to the consistency of milk and applying to the whole area. Then a second shade later on applied with a thin brush of darker shade to the deep areas and around gear.
Time consuming, but suitably moody for me.
Now that I've started using Nuln Oil I have to hold myself using that!
It's probably too watery for some uses (for the look I like anyway) but for a general purpose thing, it's great.
I do wonder if Games Workshop were influenced by Army painters Quickshade range - it the same theory really if somewhat more subtle.
Incidentally, my Citadel Inks were in pots, not the cool squeeze bottles shown in the Realm of Chaos article.
And if you're interested in trying out the old colours - Black Hat still make them