It's hard to get someone perfect for the role, but this guy isn't too far off. Ramsay has pink, blotchy skin and is "fleshy." I wouldn't say that about Rheon. This actor has the broad nose though. Pale white isn't pink, but eh.
From the video clip, his personality is NOTHING like Ramsay's. He's got the creepy serial killer attitude, but Ramsay is not like that at all. He's loud, quick to anger, and ferocious. The actor will need a complete character change for this role if he is playing Ramsay.
A number of characters in the show don't fully match their book descriptions. E.g. Allen as Theon. The important thing is to convey the character, not merely his looks. And Rheon has the kind of mug that fits a character like Ramsay perfectly, even if it's not 100% compliant with GRRM's descriptions.
As to personality, Ramsay as Reek is exactly the creepy serial killer type. Remember, he enjoys playing with his prey. It is only when he is frustrated that his wild side emerges. In ADWD he is frustrated often so this side is more visible, but in ACOK he was very cool and collected, enjoying his little game.
Personally I have high hopes for Rheon as Ramsay, even though I didn't particularly like his character in Misfits (far too timid for my taste).