GAE is any good ? if yes then JAVA or Python?
Java and Python are both excellent languages. It is a matter of taste and believe which you choose. 

If you prefer a lightweight solution, use Python.
If you have enterprise needs, whatever that means, use Java.

If you ask for my personal believe, my subjective stand-of-point is:

Use Python wherever possible and stick to other languages if there is a need to.

So this is my opinion, but as S.Lott commented on your question: 

Opinions are going to be useless.
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.