So far it's just a cosmetic feature, and I think it's going to stay that way. Unlocking achievements doesn't grant you any bonus - just the bragging rights. Unlike in NWN 2, you don't get any xp either.
I've got six done at the moment, for a grand total of twelve or so at the end. They're going to be mostly humorous or silly things, like this one.
|Achievement: Baby rat named after you.|