i like Fifita too. I don't think you want your NT to be 6'4 or 6'5. Fifita being 5'11 means his center of gravity is low, C's or G's would have a horrible time trying to get leverage on him.
2) Fifita is like 330, you want an anchor at the NT position. If you can't get leverage on a 330 lb guy, you can't move him.
3) Sure he's not a high profile guy from a big name school, but you can't have one of those at every position. The fact that you should be able to get Fifita late only adds to his value. Ferguson was a 6th round pick.
4) Like stated once, there aren't that many teams looking for a 3-4 NT in this draft. The ones that are will be looking at guys like Watson and Ngata early. The rest of the league is looking for 4-3 DTs and Fifita definitely isn't one of those.