Pencil was made to make traditional style frame-by-frame animations and that's what it does. It's greatest strength is it's simplicity. There are no extra, confusing features in the way. It's got a timeline with your keyframes, an onion-skin button and multiple layers. It's good at what it does and I've yet to find another free
program that makes frame by frame animation this easy.
In fact, lately I've been using it a lot for drawing coloring still images too, instead of GIMP or something else. Part of the reason is that I'm on a Mac; however, Pencil makes it much simpler to zoom in and out, alter the brush the way I want and even rotate the canvas for comfort's sake which is pretty unique. Another good feature is you can use the fill tool on your lineart layer and have it automatically go to the next layer down. That means you won't affect your lines and you can guarantee it won't have a weird white border. It's just a good environment for speed-painting and inking drawings (and yes you can import images). The "Preserve Alpha" setting also allows for some interesting tricks. Finally, it works well with pressure sensitivity whereas since the last update GIMP hasn't worked with pressure at all.
The obvious drawbacks are the lack of fancy tools and the occasional glitches, mostly the ones that make it LAG LIKE YOU WOULD NOT BELEIVE. And also the (thankfully very rare) bug where it corrupts a save file. But I still love it.[link]