Disclosure: Chama Valley Meat Company sent me two vaca vieja tenderloins for free and asked me to leave a review, but I'm free to share my own opinion.
I made the first one for grandpa's birthday so I could share it with a group of people. Everyone that tried it was amazed at the depth of flavor that it had. I've tried grass fed beef from several different sources and they didn't compare to the vaca vieja from Chama Valley Meat Company. I also expected the meat to be tougher because it was "aged on the hoof," but I did a side by side comparison to a store-bought choice grade tenderloin and the vaca vieja was more tender.
The other big plus is their butcher did a perfect job on the trim. There was no silver skin or anything that needed to be removed. Just add some salt, and throw it on the grill. You can say that for the store bought stuff.