If the above embedded video doesn't work see James Gurney's gouache sketching video here.
My friend Diane sent me a link to this wonderful 3 minute video of James Gurney doing a plein air sketch in his sketchbook using gouache. He is amazing. He removed two portraits he wasn't happy with from his page, put in a background tone and then painted…well just go watch the thing.
I think gouache is amazing, and fun to use.
But when you see someone like Gurney whose talent is so superlative work with it, well you just feel happy.
You might also want to watch this video if you don't understand the fuss everyone is always making about "values."