What a great puzzle - one of my favourites so far.
I got the identity of X almost immediately. Then a couple of lunchtimes codebreaking at work resulted in the decrypted message. Its really thrilling (almost magical) as the message gradually unfolds.
That was way back in June however. We'd been planning to do this as our 100th cache, but sadly couldn't find it. Now we know why We'd been searching about a mile away We were working with just a Google map (no GPS) and had input the wrong coordinates. Using a GPS today we found it fairly easily after walking a lot more than expected.
A well stocked and impressively sized cache.