I've been using kdenlive with wonderful success with ubuntu 10.04LTS. I recently upgraded to 12.04 precise, and had to work through several rendering issues.
1) Random crashes when rendering (rendering aborted). I could get lucky and get a short file rendered, but any video over a few minutes in length would always crash. The crashes seemed random--different file sizes on different attempts. I tried several output formats. I first tried the standard ubuntu packages, then sunab's ppa kdenlive-svn (kdenlive-release ppa apparently doesn't exist for precise yet!), and finally my own build of everything using the very nice build script (http://www.mltframework.org/twiki/bin/view/MLT/BuildScripts). No luck for any...random rendering aborts.
At this point, I decided to go nuclear and start with a fresh 12.04 install, this time using the 64-bit distro (had been on 32 bit). I've had much better luck with the 64-bit ubuntu ... it seems to be the way to go. I'm using the packages in the standard ubuntu repos (kdenlive 0.8.2.1-1ubuntu1 and melt 0.7.6+git20120204-2). Although when kdenlive starts up it says that melt is 0.7.7, not 0.7.6, which is odd.
2) Next, I repeated the issue mentioned here: http://kdenlive.org/forum/rendering-results-emtpy-file-when-transaction-.... Rendering immediately stopped with no error. I removed my title clip and replaced it with a png created in gimp. Easy workaround.
3) Next, I experienced a memory leak during rendering. After a couple of hours rendering a long video I was completely out of memory and swap was full and the whole system became paralyzed. This seems to happen only when resizing the video during render. My project was 1920x1080 and I was rendering a smaller version to 1280x720.
Finally I've had success rendering my 20 minute video to H264. My source is AVCHD MTS 1080i from a sony camcorder, which I previously re-encoded to DNxHD 1080i on import to kdenlive. Output to 1920x1080p is working, and the quality is great. (I'll just use Handbrake to make my smaller 720p version...)