I would jump on that deal. The Prima class ship Haven is very comparable to Retreat, in fact I think the Haven cabins are a bit of a step up in design. The only difference between the two is that the menu in the Haven restaurant is the same every night, whereas the Haven menu changes daily. However, I believe unlimited dining package is included with the Haven, so you'll be able to dine at specialty restaurants each night.
I definitely do not consider NCL on the same level as Carnival. It does have some of the gimmicky bells and whistles like race car track, but the food is excellent and shows are great. Just sailed Prima last December and it is now one of my favorite ships, tied with Edge-class. NCL deposits are refundable, so really no risk in booking.