I tried it for myself - it's a pretty standard Eclipse plugin install:
There's an Eclipse tutorial for how to setup a project; it took me about 5 minutes. I got stuck in a few spots, but figured it out. I plugged in a basic Fieldset extension and look at the depth of auto-complete - it recognizes ExtJS object hierarchy in great depth:
There's also Eclipse help covering the great range of features. Overall, I was very impressed.
I use Notepad++, but this just might sway me to use Eclipse. Although, the $995 for the cheapest bundle option, is rather stingy. I really wish Sencha would offer this as a stand-alone product.