I think for Vaul's Wrath batteries there are two main uses:
1) Shadow Weaver - this makes for a very cheap heavy support unit that puts out semi respectable AT fire. It'll be about as effective as a few guardians at shooting at marines, with the advantage of being easy to hide. I think having a couple of these guys to round out a brigade and give you some backliners that can hide entirely out of LoS while camping objectives or seizing ground can be a reasonable choice. They won't earn their points back, but they'll do something at least.
2) D-Cannon - the D-Cannon is almost reasonably priced. Basically, you get a gun that's like 2 Wraithguards stapled together, with twice the range. the downside is that the effective range is only slightly longer than the wraithguard range, because the D-cannon wants to stand still. However, if you're willing to take a penalty, it has more reach. It can also shoot through buildings and such, ignoring LoS. Compared to 2 Wraithguard, it's far less durable, worse in assault, vulnerable to anti-vehicle weapons, and so on. if you want this thing to outperform wraithguard, the wide radius of its threat area or the LoS-breaking ability has to matter
3) Vibrocannon - quite bad, don't take. one of the few things in the codex that I think is just not useful.
Swooping Hawks - I actually think these guys are great, particularly with Doom support, but also in general. you can drop them in, cause trouble for the enemy, zip around, nab objectives, and force him to redirect units and resources towards them. Some anti-infantry firepower has decent range (kastelan robots, assault cannons, Heavy Bolters) but a lot of it is short range (hurricane bolters, etc) or multipurpose (LRBT shooting). A lot of times, their lack of objsec isn't even a huge problem because they'll be dropping into something the enemy doesn't have. These guys are really great in Maelstrom or ITC (if you play that).
Warp Spiders - kinda fill a similar role to swooping hawks, but less firepower (basically) in return for far more durability. I find that these guys are nice as Alaitoc, because the enemy has -2 to hit them, and they have good armor saves. The trick is to drop down within 12" of a particular enemy you want to kill, but far from most of the enemy army and in cover. That way, you can blast down one enemy unit, but now you're 24" away from all the enemies, they're hitting you only on 5s (or maybe even 6s!) and you're making a sweet sweet 2+ save against anything that gets through. In this regard, they're extremely annoying.
I've found powerblades a little underwhelming, but since these guys get assaulted sometimes it's not so bad. A small squad kitted out for dakka is good.