iTubeX is an HTML5 and Flash video downloader for the Mac. It allows you to download pretty much any type of video from video sharing websites or any website for that matter. This app doesn't ask you to paste a video URL. Instead, it works like a web browser, which you can use to browse any website. Once you have found a video that you want to download, you can click on the "download" button at the top-right part of the screen. Three options will appear: Download, Extract Audio and Copy URL to clipboard. If you choose one of the first two options you will be asked where you want to save the file and a progress window will appear.
In my little time with iTubeX, I didn't experience any problems and the downloading was very smooth. I liked that I could use it to browse YouTube.com and download any video I wanted, even while I was watching it. You can add as many videos as you want to your download queue.
In terms of speed, iTubeX seems to be on par with other solutions out there. The app seems to download the highest quality version available, so it can take a little longer for a video to download than it takes to load in a web browser.