Love the way everything about a product/project can be included in my Assembla space. All the source code, check-in history, requirements, etc. There's a terrific Lean/Agile workflow system with the cardwall.
Room for Improvement:
The search could still be improved, but overall it's a very solid product.
We've been using Assembla for three years and it continues to get better. They transitioned to their own data center and the reliability has been greatly improved. We've had a tremendous return on our three year subscription. I would definitely use it again for a new project.