The last time I used ExtJS was a few years ago, so it may have changed but: Its tables all the way down. Horrible html generated from a really clunky declarative library. If you only ever _just_ want to make a windows 2000 deskop-like experience for older browsers, than yes it is serviceable. Also very painful to extend / create custom work when you're dealing with that level of nesting. ExtJS solves an old problem that you very likely will not run in to today. Better to use modern-ish libraries and use SASS or LESS to get that good 'ol early 2000s desktop look.
ExtJS has two modes, modern mode is based on sencha touch and uses CSS-based layout, classic mode is based on the older ExtJS rendering model and uses tables for layout. You get a richer feature set in classic mode, but the end goal is to achieve parity and then phase it out.
