I've used them a few times, and not had any issues. Here's an example.
I used to drive an '02 Expedition with the 5.4L V8. The OEM plastic intake manifold cracked at 150k miles. I replaced it with Dorman, which I found to be a better-made part than OEM. It was more heavily built, especially around the thermostat housing. All the factory brackets and clips fit perfectly. The only thing it didn't come with was the factory sound deadening foam on the underside. But I couldn't tell any difference in sound (and wouldn't have cared if there was a difference). Put 60k miles on it without issue before I sold the truck. Cost about half of OEM, if I recall correctly.