My new thing for March wasn't really a totally "new" thing - I bought a full set of Contrast and a full set of Speed Paints (from Army Painter) and really experimented with them (mostly on the #MonsterMarch figures). I'd used them before but now this let me really get to grips with them and understand. So, here are my thoughts;
1) You have to be careful with applying them - while you can get nice effects going over various colors (or other Contrast / Speed Paint) with them, they are translucent. So, when applying the main color you need to be pretty careful - or touch up afterwards.
2) You need to apply them in one single go - if you take too long, they start to dry and the effect is lost.
3) You can't miss bits - touching up afterwards to catch bits you missed is hard. It doesn't give as nice an effect.
4) Some Speed Paints (I didn't notice it with Contrast) get reactivated with water or wet paint - which can cause a problem. They need a quick seal between coats (which sort of negates the "quickness" of the technique, in many ways).
5) Because you don't water them down, Contrast is better because you can use it from the pot (while Speed Paints are in dropper bottles). There isn't a need for a palette, but you have to use one with Speed Paints.
6) Exactly as they are designed, they encourage bold and quick painting - but the end results are perfectly serviceable. I am sure a better painter would be able to get better results using traditional methods, but the results I get are just as good as anything else I have ever painted.
=][= Danforth Laertes