It's not illegal to download videos for personal use. In some cases, it might also be legal to download videos for commercial reasons, as long as those reasons are covered by 'fair use.' You may be breaking YouTube's Terms of Service by not using the download button they provide for specific videos. Still, we've never heard of anyone being banned or getting in trouble for this. We believe that downloading content from a video sharing website to watch it at another time or on a device without an Internet connection has the same purpose as timeshifting and should be considered fair use. This precedent was set in the Betamax case. As such, downloading non-copyrighted videos is entirely legal. We recommend that you always request permission from the copyright holder to download any video that's not licensed under Creative Commons.