First, a gratuitous plug for my Let's Play/Drown Out video series, currently focusing on 3DO console titles: http://www.youtube.com/playlis...
Why is that link relevant? Because they were all made using Kdenlive.
When I first started mucking around with digital video, I tried a bunch of free/libre packages, and formed the following opinions of each:
Windows Movie Maker
Yes, $(GOD) help me, I gave it a serious try. To my utter surprise, it mostly worked and did what I wanted without crashing. However, the UI was rather inflexible, and I needed more than the handful of features it offered, so I kept looking.
Every Google search for free video editing software always turns this up, so I tried it. Then, ten minutes later, I had to stop trying it because it kept crashing and/or hanging at the slightest provocation. It has an impressive-looking array of features, and the editing timeline looks quite powerful. Evidently, you can do some fairly impressive things with Cinelerra, provided you can identify and avoid all its weak spots.
The last time I tried this, it was unreliable, under-featured, and incredibly slow. Just loading a one hour-long video clip into the timeline took several minutes as it tried to generate thumbnails and an audio waveform for the clip.
Assuming I'm remembering this package correctly, all it does is assemble edits -- that is, you can tack together a bunch of clips one after the other to create a larger work. If you want to do any effects or titling, you're SOL. Perhaps the Kickstarter-funded upgrade will yield some improvements.
I had to learn something the hard way with this package: This is a professional package. By that, I don't mean it has a ton of features (although it certainly does). I mean it expects a certain level of media asset before it will operate on it in the manner you expect. Us mere proles are satisfied to use MP4 or MKV or ($(GOD) help us) AVI files. However, in the pro space, you have files that contain not just compressed audio and video, but also timecode. And not just timecode measured relative to when you last pressed the RECORD button, but also a master timecode from an achingly accurate central timecode generator fed to all your cameras and microphones. This not only means all your cameras and mics are in precise sync ('cause otherwise their internal clocks will drift relative to each other), but you can trivially sync all your master footage and then intercut shots without even thinking about it. Also, near as I can tell, there's no such thing as inter-frame compression in professional video. Each frame is atomic, which means you can cleanly cut anywhere, but it doesn't compress anywhere near as small as, say, H.264.
The result is that, if you don't have equipment that generates all this metadata for you, then you need to convert it from the puny consumer format you're likely using. This means having truly monstrous amounts of disk available just to store the working set, and tons of RAM to make it all work. And hopefully your conversion script(s) didn't cough up bogus timecode.
So, yes, Lightworks is very very nice, if you have the proper resources to feed it. I don't, so I've set it aside for that glorious day when I get some proper equipment :-).
Kdenlive is built on top of the MLT framework, and is about the best and most reliable thing I've found out there that doesn't cost actual money (either directly or indirectly). It has a non-linear timeline editor, it supports a wide variety of media formats, and it has a modest collection of audio and video effects (almost none of which you will use).
One of the more amazing things Kdenlive does is transparently convert sample and frame rates. Without thinking about it, my first video involved using a 44KHz WAV file, a 48KHz WAV file, and a 44KHz MP3 file, with the output audio to be 48KHz AAC. I feared I was going to have to convert all the sources to the same format, but Kdenlive quietly resampled them all when compiling the output video file, and everything came out undistorted and in sync.
Kdenlive does occasionally crash, which is annoying, but it has never destroyed my work. It has a fairly robust crash recovery mechanism, and you may lose your most recent one or two tweaks to the timelines, but you won't lose hours of work.
Kdenlive is not perfect, of course. It has limitations and annoyances that occasionally make me search for another video editor. But if, as I was, you're new to video editing, it will take you a while to find those limitations. Kdenlive has certainly served me very well in the meantime, and I think it's the most reliable, most capable, and most easily accessible Open Source video editor out there.