I have used YouTube Downloader HD for few days but my experience was very bad so I never used it again. It crashes a lot not only this it sometimes hangs my phone also. So I uninstalled it as early as I could. The downloading speed is also not very good although I have 8 MBPS wifi connection but the downloading speed never went above 500 KBPS so that was really frustrating. Sometimes a paused download starts from beginning so my internet data was also wasted. You can not download 2 video at a time because when you do so than the downloading speed of 1 of the either video will decrease very much.
I recommend using another app for downloading YouTube videos as YouTube Downloader is not worthy of trying.