They're 30" S3 AP6 Salvo 3/4. So, lasguns with one more shot and much less range; they look cool as hell, but they're pretty much the worst gun in the whole of the Heresy - especially in those squads that can take them... Which can take Volkite weapons for the same price; weapons that actually stand a chance of killing things.
They're even worse when you consider that a standard Tactical marine in HH can make a 'Fury of The Legion' attack, and shoot twice with his boltgun (forgoing shooting the next turn), meaning that even a Marine with a boltgun is essentially equivalent to a Rotor Cannon (which may have 3" extra range, but remains only S3 AP6, which are, frankly, joke stats for Heresy-era gaming).
The only squad where they're relatively worthwhile is for Servitors, who get them for pennies, and can form a nice (relatively) cheap bodyguard for a Techmarine Covenant, or act as a fluffy little unit if you're doing stuff like the war for Mars.
But yeah, rules-wise they're complete turd. Just complete and utter turd. They're, like, Pyrovore-bad.
The one exception to this would be Imperial Fists and Blood Angels, who can take a Heavy Weapon squad, and swap out the Heavy Flamers for Iliastus-Pattern Assault Cannon (like 40K Assault Cannon, only they jam and become permanently unusable if you roll three or more 1's when rolling to hit). So you could then take these, and use them as Assault Cannon in Heavy Weapon squads, at which point they go from being turd to YES YES NOW NOW PLEASE ALL THE NOW
(As a side note, this means that Imperial Fists and Blood Angels can get Furioso Dreadnoughts with twin DCCW and two Assault Cannon for very few points indeed. Which is both bonkers and awesome, and hasn't been FAQ'd yet, to the best of my knowledge.)
As for the weapons here, the Rotor cannon looks ace (which just makes its horrible rules that much more galling) and the Space Crusade Missile Launcher is stunning.
Not overly sold on the other things.
Yeah, those 2nd Ed shoulder-mounted Heavy Bolters look ace. Especially the original Scout ones...