I use it occasionally. For me it works well, as long as I don't use any brushes. That's because dragging while clicking isn't very smooth and you need to press quite firmly. One could possibly circumvent the problem by changing the settings of the trackpad to "click by tap" but I find that to be really awful. It results in a lot of accidently moved files and icons.
Furthermore, when trying to paint I run out of space quickly. The trackpad is just not large enough.