As the creator & maintainer of a large ExtJS application (stockor.com/demo), I find it very much a mixed bag.
Love the comprehensive widgets that are all integrated with the data sources. Back in the early days of SPA architecture I'd say they were far above everyone else.
Hate the slow, JS driven layouts. Something that's not immediately obvious when using ExtJS is that all layout is conducted by it's own engine and applied via inline style tags which set the width/height of each element. This might have made sense back in the IE3/4 days (when I started using it), but I find it really sucks now since you're hobbling the browser and not allowing it to do what it does best - Laying out webpages.
And there's the perennial bugs. Sencha is attempting to re-create the entire web ecosystem themselves, so it's no surprise that they don't quite pull it off seamlessly. I won't elaborate on this too much since it's pretty easy to visit their own forums to get a sense of the frustration level with this.
I'd posit that ExtJS made quite a bit of sense back when IE4 support was still a big thing, but I'm pretty sure it's best days are behind it. I'm sure it'll hang on for quite awhile in corporate environments, but I don't see any reason to use it for new development.