Easier than ever.
Here's a short story of some random guy (me) who started with QML two months ago (and still can't do anything).
The recent KDE Software 4.9 features a new "Now playing.." plasmoid, rewritten in QML. It shows you the current song that's been playing and allows some basic player controls, like pausing and seeking. When you put the plasmoid in a panel, it shows just one line of text, where there are three strings cycling - track title, track artist and track album. These strings were just switched on a timeout, it was a sudden change, which very oftern caught the corner of my eye, thinking there's something happening in the panel. It annoyed me so much, that I actually downloaded the source of the plasmoid and decided to add a fade effect between the string changes. Took me 63 seconds to locate the lines I needed to change. 2 minuts of Googling to find the best approach. 7 lines of QML code and one execution of "plasmoidviewer -f horizontal". In less than 5 minutes I was done. And the plasmoid looks 140% better (in my humble opinion, at least). You can see for yourself in the upcoming 4.9.1 release.
I'm actually so excited from this tiny change that I had to write a blogpost about it (along with spamming half of my contact list). Improving QML plasmoids is really easy. Don't be afraid of it. You don't need any special tools or development environment to get it done. All you need is the source code and Kate/your favorite text editor. And 5 minuts of your time.
(Just keep in mind that all your changes should go through Review board first ;)