So I've used the Heavy Xbow yesterday and in my opinion it rocks. One of the most important things that seperates it from the Arbalest:
When you shoot an enemy you do not expect him to die in 1 shot, but you are happy if it still works! With the arbalest I always hope, during each shot that I 1 shot people but it hardly works.
The faster reload time is so great! It saved my ass a couple of times, I was able do dodge some couches and reload the xbow before the cav came back at me (works ofc not for arabian). With 13 bolts I still managed to get to the end of the rounds (if I survived) with 2 bolts or so left. It is tempting to shoot a lot more because the possibility is there but if you choose your targets carefully it will work out. Its also great vs. HA if you have a little cover, when you have no chance with the arba to reload, the heavy xbow has good chances to be ready in time.
1 thing I had problems with is to adapt to the slower missile speed, missed quite some shots because of that. But thats just a question of time.
So overall, I'm a big fan of the Heavy Xbow.