I've worked on two big projects using ExtJS and both of them were/are nightmares: the code is complicated, tied very deeply to the ExtJS infrastructure, if there's a bug you wait on Sencha and you are probably going to have to pay for an expensive consultant at some point.
Now, I'm pretty negative on client-side MVC anyway (see intercooler.js) so I'm probably not a fair judge, but if I were starting a project today that had to have a large client-side footprint, I'd avoid ExtJS.
I will say that ExtJS looks great OOTB and comes with a ton of widget functionality when compared with other options, so if you want to get something done really quickly with, say, a tree UI, that might be a reason to look at them harder.
Now, I'm pretty negative on client-side MVC anyway (see intercooler.js) so I'm probably not a fair judge, but if I were starting a project today that had to have a large client-side footprint, I'd avoid ExtJS.
I will say that ExtJS looks great OOTB and comes with a ton of widget functionality when compared with other options, so if you want to get something done really quickly with, say, a tree UI, that might be a reason to look at them harder.