I think some field testing of this new weapon, as compared to the pulse weapon, will be required. I see a few things that I like about the design of this weapon.
1) Range. Obviously, in normal 40k, this isn't going to matter, but in Apocalypse, even 72" sometimes feels a little short. 120" is definitely something that will cover most of a 6'x8' board.
2) Blast: This means that against the targets you're likely to aim at, you are probably not going to miss. I think that rule is less of "shoot once" and more of "you won't miss big targets". And of course, wrecking and squad that just arrived by deep strike and didn't have the smarts to run (if you've nothing better to do).
3) AP 1: For those moments when you don't get a six, a better chance of still doing more damage.
4) Mass Induction Targeting: I think this will be a devastating rule. As someone pointed out, this means only a 1/36 chance of failing to do something. And about a 1/3 chance of getting that sweet six.
5) Cluster shells: This may be the most useful aspect of this weapon over the pulse ordnance multi-driver. The weakness of the pulse weapon is that you have to choose which shot you want. WIth this you get both! Individually, they have weaknesses against their counterparts, but the fact that you get to shoot them together I think will show itself to be very powerful.
Last thought: It strikes me that the pulse and rail weapons serve a different purpose. The rail weapon is for one-shotting enemy superheavies (without the need for lots of markerlights like the stormsurge). The pulse weapon is for dealing with deathstars.
More importantly, one can use the rail weapon in the ITC. Bwahaha!
All in all, I'm exited and I'm getting one of these weapons. If only to finally have a mainstay rail weapon back in my army!