X plan pricing varies with each vehicle. Typically it is usually a few hundred dollars on either side of invoice. It is printed on the dealer invoice, as well as A plan(employee) pricing which is a couple or three percent better than X plan.
I've used X plan on two or three vehicles over the years and have been satisfied. Just ask the dealer before anything else whether they accept X plan for the particular vehicle you are thinking of purchasing. They don't have to accept it, but almost all do. No negotiation, no hassle. You still get all the rebates, etc. available at the time of purchase.
But there are those customers who won't use X plan because they enjoy the heat of the battle. They will always claim they got the vehicle for less than X plan. Maybe, maybe not.