Well I like the fact they kept the design from DMC4 for Dante's regular Devil Trigger, and like Foxy said, it can actually fly this time which is a huge improvement since DMC4 was the only game Dante's demon form couldn't fly or glide in any way.
Dante's Sin Devil Trigger form is awesome, it's replaced Sparda's true form, as my favorite demon form
V's Devil Trigger, I guess it make sense since he's suppose to be Vergil's human half, with just a little demonic power left, so I think it was nice that they did a nod to DmC Dante where his hair's usually black but turns white when DTed.
There was some concept art for Vergil's regular Devil Trigger, which I personally saw as an indicator that Vergil would be playable since otherwise it meant the form would go unused. Unfortunately it seems so.
As for his Sin Devil Trigger, one thing I liked about both of the Sin Devil Triggers was how draconic they looked, and Vergil's version? With the silver and blue color scheme, made him look like a type of ice dragon
I do like Nero's Devil Trigger overall, though part of me does wish it looked a bit more like the DMC4 concept art. Also, the extra long hair looks kinda silly. It looks awesome regardless