I've used it, but not as the total base malt. I made a red IPA a while back that was about 20% Red X. To me, it didn't add much color or flavor in that small of a %. However, I recently brewed another batch of Grapefruit Irish Red IPA. For the base malts, I went with 5 pounds of NW Pale Malt and 5 pounds of Red X.
I'll be kegging it soon. I'll let you know how it turned out. I've attached the recipe.