Here is the way I use these words:
 
There is a subtle difference in meaning and usage.
This letter/book/ flower is for you. (A general statement - it �s for you and I �m giving it to you) 
This letter is to you. (The letter specifically has your name and address on it) 
 
Your second sentence is common usage: for me doesn �t sound as natural
This exam is important to me. 
 
To use for me I�d say:-
It is important for me to pass this exam.
 
It is important to me to pass this exam. (this is also OK)
 
Thank you. Your help meant a lot to me.  ( for me can �t be used here)