Fast Attack is a slot that went from being usable, if not exactly impressive, to being excellent. You can't call it the best in the codex because Elites still has your tank-hunters, but it's an excellent place to find (surprise, surprise) fast assault units to keep the enemy busy. Unfortunately, that's only true for the top couple of units in the slot- from there is goes downhill VERY quickly, bottoming out with some of the worst units in the codex (though admittedly still better than the poor, benighted Pyrovore.)
The FA slot is mostly about supporting the rest of your forces with aggressive attention-drawing units. Rarely will they survive the battle, since their job is essentially to crash headlong into the enemy formation in order to delay them, but that isn't really a big deal, as expendable units are just fine for the Hive Mind.
Gargoyles
By far the "best" choice in the codex nowadays, Gargoyles give you a fast, threat-projecting element that can screen other element of your force in a pinch. At practically the same price as Termagants (but unable to get free Toxin/Adrenal) but significantly faster and better in combat, Gargoyles can be a serious pain for many armies to face early on, as they can get up close very quickly and clog movement/shooting lanes, giving the main body of of the army time to arrive.
However, they are not without their faults, which (along with the annoyance of transporting the models) is why I have wavered on them. For one, with the new cover rules (needing 25% obscurement for infantry models), Gargoyles are sometimes hard-pressed to get a save against anything, especially with their tendency to rush forward at full tilt. This same behavior also can mean that they are put out of Synapse range in the early turns, likely causing them to do something annoying like race for cover or pew-pew worthlessly at a tank.
However, for the aggressive Tyranid army, Gargoyles are a real godsend and running 40-60 of them will be fairly common. Gargoyles should always be equipped with Adrenal Glands to ensure that they can charge/hurt tanks early on- fifteen or so of them will generally be enough to glance a Rhino to death, which may not be an amazing use of them, but all those squishy bodies will still be getting in the way and getting in the enemy's face. Toxin Sacs can certainly be worthwhile if you have some points available, but I don't consider them a high priority- rerolls to wound are nice and all, but killing models is not really your main goal. You want to get stuck in and be annoying- anything you kill is just a bonus. Broods should never be less than fifteen models strong, as you simply crumple too easily under firepower; twenty is probably ideal at lower values, going up to thirty in 1750 or 2K.
Raveners
Though suffering significantly compared to Gargs, Raveners may still have a place in the arsenal because of their rare ability to ignore cover combined with excellent speed. On the other hand, 5+ cover combined with T4 is a recipe for disaster against the many, many heavy weapons out there, which is a major limiter on what they can do.
Beast status is now a pretty huge deal, giving you 12" move and Fleet as well as ignoring all cover at all times, giving Raveners effective assault grenades. Though they are not particularly deadly, a significant number of WS5 attacks with Rending will at least not just be shrugged off by most Marines. Raveners are quite capable of chewing up small squads of MEQs or GEQs in significant number with relative, as they have a fairly good set of numbers behind them, but the 5+ save will usually give them trouble if anything survives to swing back.
Raveners are likely a niche choice at best, but unlike their unlucky Warrior cousins, they seem to still be a reasonable pick in the right army. Rending Claws are pretty much a given for them, though you shouldn't feel too horrible about taking running twinned Talons. The various guns are likewise okay but not awful- if you go with one of them, Devourers or Spinefists are probably best because of cost.
Harpies
Now, unfortunately, we start to get into the more mediocre options. Harpies benefited a lot from some of the new changes, specifically becoming Flying Monstrous Creatures, but with a pathetically-weak Vector Strike and low survivability otherwise, their place in "real" armies is questionable. I do love the flying doofuses, but I can't in good conscience claim that they are powerful or effective.
Being hit on sixes is a pretty excellent place to be, especially combined with the maneuverability options of a FMC and ability to start on-board in all games, but with flyers running rampant in the game, everyone and their brother is bringing ways to shoot down such targets, and as an FMC you have a risk that comes with going high- Grounded Tests. Every time you take a hit, you've got a 33% chance to take another bonus wound on top of it, and unlike Daemons you don't have a save to shrug those off with. Add in your mediocre regular save (4+) and utter inability to shoot at enemy flyers and you're looking less and less viable. Oh, and don't forget that the change to the damage table means you can't kill a tank unless it's open-topped.
As a result of all of the above factors, the Harpy is something that should be left in the box unless you are looking for a kooky conversion project, a gimmick FMC army, or have never seen an Autocannon or Heavy Bolter in your entire life.
Spore Mine Clusters
A very cute gimmick, but not one that really ever works due to their awkward deployment rules. And since pretty much all of them will vanish as soon as the game begins, a pretty expensive one. Servo-Skulls cost 3pts and do basically the same thing, except they actually start where you want them to every time.
Skyslasher Swarms
These are like Rippers, but more expensive for essentially no benefit. Oooh, they can fly- that means fewer cover saves as I rush in to get into combats I can't win.
Shrikes
The winged Warriors suffer just as much as their compatriots do from the changes to cover, all the more so because their natural save is weaker and they are more likely to be in isolated positions. They can provide forward Synapse, but unlike the Parasite they can't really hide anywhere, so they will generally be shot down quite quickly.
Though hardly as bad a choice as Sky-Slashers or Spore Mines, they're not something you will really ever see on the battlefield because they just don't have much of a role. If they were some sort of non-Monstrous FMC, on the other hand...
