They're not awful and thanks to their rangefinders they can be the bane of skimmers and bikes alike. Sadly the suffer the same problems that Space Marine devastators do. They're far too static. Now it depends on how you build your list but it seems a shame to bring something so immobile to what can be a fast and flexible army. I'm sure some will find a role for them and will build their army around distracting attention away from them whilst they hammer targets. However, personally I can't see them making it into my army.
Vaul's Wrath Support Battery
The D-cannon whilst short ranged benefits from the Barrage rules meaning sniping models with S10 AP2 shots. Blasts mean the opportunity to take down several tightly packed vehicles at once but that's rare in the current meta. The vibrocannon relies on several hits to be effective so when the unit has lost a couple of models it'll struggle. The shadow weaver is pretty effective against tightly packed troops but with the FAQ setting it firmly as a small blast weapon it won't be as useful when models are widely spread as any opponent with sense will do. Personally I don't think they do much that a more survivable unit couldn't do the same or better.
It's got great flexibility which is something any unit needs to make it into a list. Think of it as a hammerhead with less strength. It's large blast may be S5 but AP3 is a boost compared to the hammerhead. For those recently arrived terminators there's S7 AP2 small blasts and an S9 AP1 lance for tank hunting. The list of things that a good lists needs to be able to deal with includes AV13/14 vehicles. The fire prism is the Eldar answer (outside of Fire Dragons). That S9 shot will be penetrating everything AV12+ on a 4+ and then blowing it up on a further 4+. Against AV14 that's actually better than a railgun (without Longstrike). Worth paying 125pts for? I'd say so. Even with holo-fields it's only 140pts. It may have to compete with some other excellent options but if you're trying to put up an AV12 wall it can't hurt to have a couple of these.
With the same basic cost as the Fire Prism the Falcon essentially trades a better main gun for the ability to transport a handful of troops. However, when you've got the excellent wave serpent already capably filling that role the Falcon is left on the sidelines. It doesn't compare well to the fire prism in terms of offensive power and loses out to the wave serpent as a transport. Both of the alternatives cost about the same so in this case it's a jack of all trades, master of none kind of thing.
That's it for the first part but the real stars of this section will be covered tomorrow so come back for that...