Make no mistake, when Warburton named this the XD (eXtra Deep) they weren't joking - it's a deep, V-shaped cup that takes considerably more work than their D cup does to play. The sound is superb - fat and rich.
As usual with Warburton mouthpieces - a fairly wide and flat rim, moderately sharp bite and tightly build to exacting tolerances (as you can tell by just how well their tops and backbores mate up).
Flexibility is superb and its easy to get the articulation nice and clean... this comes, however, at the expense of ease and security of the extreme high register.
As with any mouthpiece, you'll like it if those compromises suit what you want.