I gave it a try, downloading a song from YouTube I was wanting to add to my current playlist to burn to a CD... the results are really good, but I haven't tried an actual video yet.
One thing I will say is that I like that unlike the other sites, they actually offer you options to download a video in low quality (240p), standard quality (360p), or high quality (480p), the other sites basically downloaded in standard quality.
So, it looks like Google hasn't gotten to them yet, so I guess they'll have to be an alternative for downloading videos (though I'm still going to use Zamzar for when I have to convert files I already have from one format to another, like wav to mp3).
I do have one more question though: does Clip Converter regulate a limit of downloads/conversions per day? That's another reason Zamzar was better than MediaConverter, because Zamzar allowed you unlimited downloads/conversions per day, whereas MediaConverter recently dropped the limit down to 5 per day for free years (used to be 10 IIRC).