I've been using SketchUp a lot in my high school technology class, and my students have really taken to it. We've actually been using it to create/design structures, which I understand is not what you had in mind. I think the best use in Geometry would be in having students create 3-D solids. The students can draw polygons and use the push/pull tool to raise the 2-D shape off of the ground, creating a solid. Once the students have that skill down, the sky really is the limit. You have them create structures using certain shapes/solids.
If you have specific questions about SketchUp, let me know. I also recommend the "SketchUp for Dummies" book. It's a great resource. The author has over 60 YouTube videos about how to create houses, etc.