I haven't used Udacity personally but I've used Treehouse and Lynda to learn some specific technologies. I used what I learned to take on side projects and those projects have been impressive enough that resulted in getting interviews and eventually a job with a YC company. (I would also note that I have a four year Bachelor of Science in Computer Science).
I've never considered finishing their courses as a certification that would entitle me to a job. So whether it's worth it or not it all depends on what you get out of it. For me, I wanted to do something specific, their courses covered what I needed to learn to do it, so I took them and used what I learned to do what I wanted to do. What I ended up building with my new found knowledge got me the interviews and eventually a job. So you can say it was worth it for me.