Welcome aboard
1) The Helblaster's popularity has decreased dramatically in 7th edition, but I'd say is has risen again somewhat. I seem to recall that some people here fielded one in their armies.
2) If we ignore misfires for the moment, one salvo will give you 18 shots on average, equaling 9 hits on short range and 6 hits on long range if shot at an ordinary target. Add 3 if firing at a large target, subtract 3 if firing at skirmishers. As with most warmachines, misfires are a major drawback. Since all those shots are S5, the Helblaster can be pretty effective.
3) Tough question. In terms of killing power, you have to aim it at something expensive, like heavy cavalry or a monster, to get the points back. It depends on your enemy whether he fields such a target or not. But the Helblaster also has a psychological effect: since it is potentially devastating, many people will hesitate to move expensive units into its firing arc. This is something you can use for your advantage.
4) If you have only one cannon now, get another one, if you have two already, you should - in my opinion - rather take the Helblaster if you can spare the points. Others will disagree with me here.
The Helblaster is quite different from a cannon. Unless getting a flank shot, a cannon will not destroy a cavalry unit, whereas the Helblaster has a decent chance to do so. What's more, the Helblaster is actually more calculable than a cannon, since you throw more dice. It also doesn't take a special slot, and special slots are precious in an empire army. On the other hand, a cannon is the best way of destroying enemy war machines, has a far better reach, and is a sort-of template weapon. Two cannons is always a good idea, and most players here seem to field this combination.
