Having met Krita for the first time, and with a technical understanding of Photoshop (I'm really a software developer by day), I found it fairly easy to use, and could get some good (in context!) results from it quickly. I'm using a mouse, but haven't yet learned brush customisation, and think that's the key to getting what I want out of it.

I've been learning by giving myself a small canvas, and 5-10 minutes to develop a scene. Then I start again, and see if I can do better with the same scene.

http://johnvalentine.co.uk/index.php?art=art
^ also contains 40-minute works.

That's some awesome work with a mouse!

I used a mouse in Photoshop for years. When I downloaded Krita, it was obvious that I'd never be able to paint the way I wanted to until I got myself a tablet. For my birthday I bought myself a Huion H610 Pro V2 w/tilt. I'm still struggling with Krita but the first painting I did was acceptable...to me, at least.