Since I used Quickshade several times by now I want to share my experiences in a short article, because in my opinion it is a very useful tool if you have to paint a large number of miniatures.
Quickshade is basically varnish with pigments mixed in. It's produced by Army Painter and available in three variants - "soft", "strong" and "dark". The variant determines how many pigments are mixed in. The can is 250ml.
Using it is very simple. You paint the miniatures in basic colours, without any shading or light effects. Then they're ready for Quickshade. You can either dip the miniature into the can and shake off any excess paint or you use an old brush to apply a coat all over the miniature. I prefer to use a brush since dipping can create quite a mess and the only miniature I tried that with ended up with excess paint at unfitting spots. Further, my second try - a lemon made from Fimo - is still in the can because it fell in, so dipping is not a good solution for clumsy people. :P
It also wastes a lot of paint. The downside of brushing is that you need brush cleaner afterwards since it's varnish, after all.
Once shaded the miniatures require about 24 hours to dry. After about 12 hours they can be touched but they need the full 24 hours to be completely hard. Quickshade is very shiny so you can add a layer of matte varnish afterwards to give the finished miniatures a more natural look again.
(Mini review in the mini review: I strongly advise against using the Antishine spray varnish from the same line of tabletop products! Silly me used this on a very large, very expensive and very time consuming model first and after drying the varnish had a coarse, sandy surface kind of like sanding paper and the model looked dusty all around. It was awful and we disposed of the spray can right away.)
Let me demonstrate the effect on two zombie figures from the "Zombicide" board game produced by Guillotine Games/CMoN.
First I painted them in basic colours, including the gore. Since the zombies typically shuffle through streets I kept the bases in simple grey with some splatter.
Afterwards I brushed on a coat of the "soft" Quickshade from the well-stirred pit. When working with a brush you always have to be careful not to create any air bubbles in the sealant since these will pop while drying and leave ugly effects.
The pigment will automatically sink into the folds and creases of the miniature so that you end up with distinct shading when it's dry. Keep in mind, though, that the whole miniature becomes a bit darker from the sealant. So if in doubt paint it in slightly lighter colours than desired so the end result will match your taste. Additionally, be careful not to overlook any parts of the model since gaps in the sealant will be blatantly obvious after drying and it's almost impossible to repair since several layers of Quickshade add up in pigment. In case you want to add highlights in addition to the shades you have to paint these on BEFORE using Quickshade, else the colours won't match. I did this successfully on small parts like hair but also on a whole horse model and it looked good.
As long as you apply the sealant evenly the shadows will appear at the right places.
Naturally, this isn't as perfect and pretty as shadows that are painted on by hand, but it is an enourmous time saver, e.g. when you have 600 zombies to paint - and it still looks very neat and stunning, which makes it a good option for beginners too who might not feel that comfortable with inks and effects. If your collection is big there comes a point where you have to set priorities. From now on I will paint basic troops and plastic figures that are used in dozens on the gaming board with this method and only the special models and heroes will recieve the full painting effort.
I have to mention though that Quickshade is only for those who like their models to have a certain "grubby" look. In the end I didn't like Quickshade that much on my high elves, since brown shading looks a bit dirty, especially on silver armour. I suppose the effect won't always look that swell on colour that don't match brown, e.g. when using much dark blue. In cases like that some pondering and maybe painting a test model might be a good route. My guess is that the "dark" toned Quickshade is probably rather black than brown and that will take care of some problems with clashing colours, but since I only use the "soft" sort I can only guess. As a matter of principle, brown can only become so dark before it becomes black.
As you can see the "soft" tone is not good enough for the dark-skinned zombie - the effects on its skin are rather marginal. If many of your models have skin like this or wear very dark clothes in general you will probably do better if you buy the "strong" or "dark" tones. For me, "soft" is definitely dark enough, since I use lots of white and other light colours. Apart from the zombies I used this on a goblin (medium green skin), a unicorn (medium gray with blonde mane), a high elf (silver, white and light blue) and a Syntha (black and beige) and for these miniatures "soft" was quite perfect. One time I also used Quickshade on things I made from FIMO.
Quickshade is rather expensive but one can gets you a very long way. That I started on my second can is just due to the fact that the first one is still lost in some moving box.