Hi there,
This is tricky question.Just wanted to know the background of yii.Frameworks like codeigniter and zend are backed by established companies investing in them makes one feel secure.so how can you feel secure in investing in yii(there not much info about this on site)?
Raffi
The founder of Yii is Qiang and he is the main driving force… but Yii is an open source project and community driven… so anybody can contribute… and or continue the work on Yii…
Why would an company give you security?
A bit OT, but to make my point 
Take for example the OpenOffice project… oracle brought it… so you would feel more secure… but see what happened… all the main developers of OpenOffice abandoned Oracle as they do not agree on the company ideas… and now we have the LibreOffice…
So the main developers of OpenOffice are now working on Libre Office… OpenOffice has Oracle behind him… Libre Office does noto have a company but an independend foundation behind him…
so what would you choose here… as for me… I will always stay with the main developers… not with a company…