It depends on your project. Creating terrains, placing models or even "painting" them on top of your terrain will save you much time.
A good in-game material editor saves you much time. You will see that, when you place models in a level and when they do not fit to the rest of the environment.

Modelling and texturing is always the same. But creating real living worlds is another story. I experienced a lot what a difference it can make if your world editor reflects real-time changes. Level editing goes much faster.

At the other hand you don't need it that much if you make a 2d casual game. But even then a good editor makes a big difference. Look at the TGB editor of Torque Game Builder. You can paint your collision-zones, place all elements, graphics, preview everything in real-time and test particles. You can test and prototype games much faster.

And I agree with Amy that your architecture visualizations could be even more interesting and easy to watch if they simply run in the browser of the client.