I did the distance and onsite course with Sheffield several years ago and got the license. I believe Sheffield is one of the best school in FAA Dispatch.
The course is very demanding but teach me a lot of theories and knowledge in dispatch. You need to self study the Gleim ATP test Prep and better get a pass before the start of the course. If possible, study the two FAA Weather manuals in advance and it helps your study in Sheffield. Especially passing the ADX test before hleps you to fully concentrate on the Sheffield course.
If you just want to get a pass and get the FAA license, other schools may be better, if you want to show your capabilities and you are the top notch to future employers, Sheffield is for you. Of course, you need to pay for the advantage you get..for a hard working stay in FLL.
Good school teach your the in depth knowledge but worst school just use Test Prep as the main text book, easier to get a pass with them but quality not assured.
I agree not all the students who failed to get a sign off for practical exams are lazy or incapable...instead, they are hard working but just can't achieve the Sheffield standard. One of the Indian guy who failed the 200 hours course mentioned he spent a lot to travel to FLL, studied very hard but still couldn't pass...it's sad but unluckily for him. though I think they may be able to pass if they have more time to study but not available.
Anyway, once again getting the license from Sheffield is not just for the license, it is for your future employment too. Base on how many airlines send their employees to sheffield and it is already a concrete proof of the Sheffield's Quality.
I receommend Sheffield for any serious future dispatchers, and of course there should be other good schools in USA too, but I dont have experience with them.