It's definitely more convenient than writing the XML by hand. I can recall more than one occasion where I couldn't get the positioning of my UI elements quite right because of some attribute/value I was missing or had set incorrectly. I HATE wasting time trying to chase down what I did wrong just to move a button X pixels. It also seems faster and much less annoying than editing XML by hand.

The really great thing, though, is that it's not tied to Eclipse, which is fantastic for people who don't use it for Android development (i.e. me :) )