The end of the series will leave you feeling both sad and happy. Cody closed the series perfectly. It is not an easy happily ever after; there is still a lot of pain for both Mariah and Gabriel. It is not the perfect ending that you usually expect for characters you become attached to. But the ending fits with the whole series, and in it Gabriel upholds his one promise he made to Mariah in the beginning.
If you were let down by parts of the first two books the third one will at least give you closure.