Ok, just downloaded this, I guess it's been around for a while. I never knew about it.
The concept of having a FREE application that allows you to create anything you want, and then have Lego ship you the parts sounds like a dream come true.
The only issue I have with the application (which is easy enough to use) is the limited block sets... It comes with one for trains (as of April '07 or build 2.0).
I was looking to build something for Star Wars, but realized that even standard blocks were missing, not to mention the less common pieces.
What's MORE disappointing is that you can't get the missing blocks anywhere... So if you want to really get creative, you're stuck with just basic blocks.