Fluid Layouts & UI Frameworks

So, the time has finally come that the designer at my work has deleted the two parallel guides, 960 px apart, spelling an end to the constrained-width era of our admin system. The design I’ve been given is for an application more than a web page, and however much I like the JUI widgets I can’t hep but feel that they aren’t comprehensive enough for the sort of interface we’re striving for. Something like Mocha UI, UKI, or ExtJS (I’m aware of Ext4Yii, but am not convinced that either it or ExtJS are that much ahead of everything else to be worth the price tag).

So, has anybody had any experience integrating a more comprehensive UI library with Yii? Any tips or things to watch out for?