They do work, though. Pre-built field boxes are becoming as popular as ARF's. Just about every manufacturer has one out. Things to look for include how much room you have. Pre-built boxes are good for someone in a hurry or someone who has a hard time building.
Kits are a good choice for someone who has a little time.
They require gluing, sanding, and painting. They are usually a little cheaper than the pre-builts. The biggest advantage is you can customize them to fit your needs. Many pilots also add their own graphics. The last choice is the XL-15 class, and about the only thing to say is to wait till you've been flying for a while before you get this type of box.