ummm it is ‘qiang’ not ‘giang’ 
and no Yii is not the ‘easiest’ framework to learn, but it is easier than others and yes it is very worth learning. I have used CodeIgniter and others (Kohana, Cake, Symfony, etc…) and have my likes and dislikes about all of these. Yii was a bit intimidating at first but quickly showed it’s power and ease the more I dug into it.
I am no Yii expert yet but I can promise you I can have the shell of a web app up with all pages needed for create, read, update, delete (CRUD) records in my database in place and role based access control (RBAC) in less than an hour.
That is a feature I love about Yii. While other developers are still fiddling with with config files and other tedious jobs in other frameworks I am already deep into coding the meat of my application! Yii (via yiic and gii) has handled many of the more tedious aspects for me and allowed me to move on to some real application logic tasks.
Also, the abstraction of data retrieval & display as well as many other objects (like forms, links, form validation, etc) is a real time saver. This again makes me consider Yii ‘easy’.
Sure most frameworks abstract these items as well, but not all frameworks abstract them well (poorly documented and cumbersome methods), or sometimes they abstract them to the point of obfuscation (no docs and no obvious methods)!
One last point, Yii has a nice book written to teach you how to use it as well.
I would advise any that have questions on learning Yii to buy the book ‘Agile Web Application Development with Yii 1.1 and PHP5’.
I have read this book cover to cover and wrote the whole application that the author uses as example in the book. It was an amazing read that teaches loads about Yii and what it can do! There are some mistakes in the book, but the author (Jeff Winesett) has a page in the Yii forums showing all corrections and is very helpful in understanding the concepts in the book.
Yes, thats right he is also one of the authors of Yii so you get taught by the people that create the framework by buying this book. What more can you ask for?