Very thoughtful question. We'd be very interested in seeing client feedback.

Although we paid our architect (for designing a beautiful addition) at an hourly rate, and didn't have any particular complaints about that, I would always prefer a flat fee--whether for a designer, architect, painter, etc. I like to know up front how much something is going to cost and construct the budget accordingly. But I realize that the flat fee structure doesn't work for every type of job.

We paid a percentage of the construction budget with incidental charges for copying extra blueprints or travel to our building site. It seemed fair to tie the fee into the square footage/complexity and detail of the overall project.