I started my current project in ExtJS and a few weeks in, I switched to YUI. The ExtJS components look great, and are fairly easy to use if you use them in the standard way. However, I found that as soon as I wanted to do something custom I got totally lost in the 'ExtJS' way. While the YUI components are not quite as polished looking, I have a much easier time extending/reworking them. One thing I like about both frameworks is they provide layout management, so it is very easy to have a 100%width/height viewport layout that the framework manages (YUI's LayoutManager is in beta...so I guess I hope they don't scrap it before it reaches production level.)

I will add that the ExtJS forums are very active and pretty helpful...as are the YUI ones. I've asked several questions in both and gotten good responses each time.