I did some interweb research a couple of years ago. While sorting through the forum threads and such, I noticed one common thing....
Reloaders that use the X-die really seem to like how it works. While I may be incorrect, I don't recall X-die owners posting comments about the die not doing what RCBS claims it does. Given how people like to complain in forums, that probably says something in and of itself. Perhaps that has changed since I researched it....don't know, haven't looked since. On the other hand, reloaders that hadn't used the X-die seemed to have little enthusiasm for them.
That being said, I bought the X-die for reloading .223 Rem. I personally don't like measuring/trimming .223 brass. Unfortunately, I haven't cranked out any finished rounds as of yet. My buddy dropped out of the shooting scene shortly after my X-die purchase and gave me several thousand rounds .223 ammo so there has been little motivation for me to set up my press for cranking out AR-15 practice rounds.
Were I you, I would spend some time doing some reading up on the topic and see what has been happening in recent years regarding X-die usage. If you have the time, research is cheap. Good luck.