I installed two Andersen patio doors in my new house; they provide access to my porches. We were surprised that the units don't come preassembled. Andersen sliding doors come in several boxes that contain seemingly a hundred pieces. To make matters worse, the assembly instructions are extremely poorly written and the illustrations are nearly useless.
The fit-and-finish quality of the Andersen patio doors is disappointing too. Screwheads don't fit flush and the joints where the header piece and the side jambs meet don't fit together well. The whole frame is held together with a few screws (which aren't made of the highest quality steel judging by how easily the screw heads strip out). Even the joints on the preassembled door panels don't fit tightly -- up to an 1/8th inch gap on mine.
Many of the holes needed for assembling the sliding doors are not pre-drilled; you have to locate and drill them yourself. Inaccuracies and misalignment are all too easy. The holes for installing the door handles are partially drilled, but not drilled all the way through. Insane.
The operating (sliding) door panel is made to work with a left-hand or right-hand opening door ... which means there are ugly/useless holes on one side that must be covered. This despite the fact that you have to order a left-hand or right-hand door. Just one of the signs that Andersen cares more about cutting costs than about building a quality patio door.
I cannot recommend Andersen sliding patio doors. Their windows are nice but these things leave a lot to be desired, and the price is high. The quality is bad, but they sure are expensive!