I've used Kdenlive pretty extensively (as a hobbyist). It has an impressive collections of features. Different releases tend to vary greatly in terms of stability. I've had it eat my project (and even corrupt save files). Back up frequently if you use it. The last time I used it, it was pretty stable, so possibly they are improving in that area. My biggest complaint is that the workflow is tedious. There are absolutely not enough keyboard short cuts. If you apply the same transformations over and over again with slight changes, you're going to be hugging that mouse pretty tightly. My next biggest gripe is that the sound editing was really flaky when I was using it. I eventually gave up and did all my sound editing in Audacity. I often liked to do transformations with sound and video at the same time and it was really hit or miss if you could preview it, or if it would bug out, or if it might crash things.

The thing is, I really want to like Kdenlive because it is really close to being able to do what I want, but I just felt that every step of the way I had to be very creative just to figure out a workflow that would work well. I eventually moved to the video editor in Blender which has a lot fewer features, but the workflow is really exceptional. I figured that what time I saved struggling with Kdenlive I could spend using other tools for features that aren't in Blender.

I don't do any video editing any more (I decided video isn't really a medium that I enjoy being expressive in), so take my comments with a grain of salt.