Video DownloadHelper is much faster than most alternatives. It is reliable, though I sometimes need to reload the page to get it to connect to the needed video. Results are high quality (1080p, or the native resolution of the clip). The watermark is not an problem for me, but I have now purchased the license anyway.
[ADDENDUM] With the license, downloads are noticeably faster. Also, when a video site changes its download protocols, it seems that the licensed version adapts more quickly.