GAE is any good ? if yes then JAVA or Python?
You'll be able to accomplish the exact same results using either Python or Java.  Java is much more verbose than Python, which can make it have a bit of a steeper learning curve.  The fact that you have some experience in Django (which GAE's webapp is largely based on and which you can even use directly if you want) will make it easier for you to get the website up and running in the immediate term.
So again, it does depend on what you want to accomplish.  If your goal is to learn Java, then doing a project in Java is the best way to learn it.  If your main goal is to get the site up and running, Python will be a better choice as it will let you focus less on struggling with learning new Java syntax and more on simply getting the website off the ground.
Basically I am coding websites in PHP from last year.
But now I want to use something else and GAE looks a good option.
So I want to know if GAE is good for making a little website to share favorite youtube videos ?
I have done single website in Python+Django few months back, it looks good to me.
But JAVA is the language that I want to learn too (never coded in JAVA since School days ).
Phew, it is hard to choose, so I need opinions !!
Specifically : Want to know if any glitch/problem in using either python or Java under GAE. Or if GAE is preferred or not. Not gonna make website for learning only, it will be for a client.