I've done it from scratch for several components and it's really quite easy. You just have to understand what the Ext wants and, while their examples are lacking, their docs are good and their forum too. The HTTP stuff is trickier than the DB related stuff but if you have a good handle on HTTP, you'll be fine.
I want to go on record, however, as saying that my honeymoon with ExtJS was over about 2 days after I started really working with it. It's very powerful and pretty out of the box but it has many places where it either degrades painfully, well actually, pretty much everywhere, and points which don't degrade at all; its form handling is pure JS, you cannot plug it into an HTML form without oodles of custom code.
I used YUI a bunch before I decided that Ext was a better idea. Now, I think I was wrong and looking down on YUI just because it is so much more verbose than jQuery (which I prefer but doesn't have anywhere near the same UI plugins of Ext and YUI). Especially with what I've seen of YUI 3; like its sanboxing.
Unless you are committed to marrying an application to Ext, which means a commercial license if you'll be selling your software, and don't mind building something which will be unusable without JS enabled, I'd say stay away from Ext.
