My lovely wife got me a selection of Citadel paints from Games Workshop for Christmas; as I was in the middle of painting the Blood Rage Ram Clan, so I continued on with those paints, and today was my first time out with the Citadels. Up to now, I've mainly been using Reaper paints, with a few Army Painter and Vallejo to fill in gaps. I did notice a difference in intensity and coverage with the Vallejo paint, but the contrast between the Citadel paints and the Reapers is pretty drastic. The Citadel paints go on thin but with a ton of pigment, giving great coverage and allowing the underlying features from the mini to show through, no matter how small. The painting experience with them is much improved.
I've been reluctant to try the Citadel paints because of the price difference and because I'm used to droppers rather than pots, but after a day of painting, I'm pretty much sold. If you can afford them, I highly recommend focusing your core set of paints on the Citadel line.
Plus, who doesn't like a paint called Screaming Skull?