X-plan is a good price, but if you're a really good negotiator or get lucky, you might beat it, especially on common models. A/Z-plan is an awesomely good price (very near dealer cost minus holdback).
Don't forget that X-plan includes all the fees, and places caps on the docs fee and admin fee. It's not a good deal to say "I got $500 under invoice!" from a dealer than then adds on a $695 doc fee and $395 admin fee and $195 prep fee and then adds back the destination, advertising, and fuel fees... What really matters for comparison is total price of the car before tax/tag/title. Get that number from your salesperson, out the door, all fees included.