It's free and pretty powerful.
It's only free if you're OK with 720p output, limited input, and not being able to move your source material to a different editor. The latter is actually the bigger risk because if Lightworks goes away (let's hope not) there'd be no way to buy the 'pro' version and get your data exported.
Otherwise it's $279 or you're on a subscription plan. It's probably still the best choice available, but be aware you don't just go buy a GoPro or a Nikon and plan on dazzling folks with the HD output with free Lightworks. From what I've seen, even iMovie parity on Linux costs $79/yr.
Even if you're very frugal and can use Free, it's probably smart to buy a month once in a while and export your projects.