I'm a relatively new user to kdenlive (I've been using it for a few months to lightly edit some home videos). Recently I gave it a go for a book trailer to promote my new collection of short stories. I was really impressed, there are a lot of small touches that are very appreciated (for example, dragging the size of a clip and having it "stick" to another clip on the track below … it's a small thing, but makes you feel that the software is working with you). For this video, I used a lot of effects/transitions: crop, rotate, and composites stacked on composites (for the titles and background image). I have a decent computer, but I was still impressed with how well kdenlive kept up, especially in the real-time preview. There are a few annoyances (sometimes I had to delete a title clip and re-insert it to get the typewritter animation to work properly), sometimes it refused to let me add a composite transition, but overall those were minor and didn't detract from how much kdenlive is to use.
If you get a chance to check out the video, let me know your thoughts or any advice for improvement in future videos: