I love gouache paint. Gouache (pronounced like "squash") is an opaque watercolor. I love it because it had good coverage, dries quickly, doesn't feel like acrylic plastic, it dries with a matte finish, is non-toxic and can be re-activated just like watercolor paint cakes. The best part is I can paint with gouache like I do with oils - from dark to light.
I was first introduced to gouache at Laguna College of Art & Design as an undergrad. I took a Color Theory class with George Zebot and we had to do tons of color swatches in gouache. I quickly discovered how expensive those little tubes of paint can be! For example, a little 15ml tube costs around $15...so get them when they're on sale or you have a coupon. I usually get the Winsor & Newton Designer Gouache tubes or the Holbein tubes.
One thing to be aware of are the Acrylic Gouache paints. These cannot be reactivate when they dry. This may or may not be a good thing. I accidentally bought a few of these and they dried on my pallet like acrylics do. One positive aspect of this though is that if you sneeze or spill water on your work, the paint won't be reactivated because of the acrylic binder.
Here are a few painted sketches I've done while visiting my family in California. I spent about an hour on each one (because believe it or not, it's actually kinda cold in Cali during the winter, and my hands were freezing!)