The Warp has overtaken them (but it's a good pain for them) so hard that their capacity for reason or even basic speech is no longer present. They're little more than deranged bat-monsters with an overactive hunting instinct. Their hands and feet have mutated into wickedly sharp talons, at the cost of being unable to use any other weapon, and they are partially daemonic; in fact, they are capable of travelling directly through the Warp, cutting their way through the Veil itself whenever a good battle can be had. They are so bestial that fielding them is supposed to be the equivalent of releasing the dogs of war or the kraken- rather than being given orders, they're shown a picture of what they're supposed to kill or given a possession of the enemy so they can track it by scent. Whenever they're not in flight, they even hang upside down from the ceiling like bats or impotently crawl on all fours, having lost the ability to walk normally. They get a good depiction in the Space Shark's novel, where they are given a roost in an abandoned Imperial chapel and are unleashed en-masse to rip & tear a whole Terminator squad to shreds.
When launched in 6th, they were a disappointment. They had claws sharp enough to cut reality but not scrap metal worn by meganobz, they were expensive and they had to deep strike in order to use their special ability - and they had to get within 6 inches in order for it to work! They were basically universally panned by players at the time save for those looking for an expensive and fragile distraction. 7th gave them a minute usefulness bump by making them Troops in a Night Lords army along with Raptors, however Raptors were picked ahead of them again because of their versatility and price point.
With 8th though, the Talons have a new lease on unlife; the AP of their weapons is now terrifying to TEQs. They also get a nice little ability that whatever they charge cannot overwatch back at them if they charge on the turn they deep strike (which is an 8th edition thing now). Now that deep strikes don't scatter (with the caveat that they have to be 9" away from the nearest enemy unit), the trick is to get them into close combat from deep strike. Unfortunately, rolling a 9 on the charge roll without a khorny icon is problematic. While they're protected from the dangers of Overwatch, your chances of actually getting the charge off is fairly low. Likewise, with the new FAQ Warp Talons (and any deep striking unit) cannot have Warp Time cast on them, denying them a guaranteed charge on the turn they arrive
The thing is that they are super duper overpriced. If you want some deep strike impact, even mutilators are far better (worse: Bloodletter bombe). And fly+mvt 12 is useless when a sneeze will wipe you... For just 2A.