I would consider Math Mammoth to be an elementary program similar in style and tone to the Dolciani books. The author is not a mathematician, but her background is sufficient to do a better job than most American elementary textbook authors.
We mostly used Singapore for elementary, but I really think I like Math Mammoth better for teaching the Why's directly to the student. The key is to read all of the blue boxes and only do a few key problems in each small section then focus on puzzles and word problems. Otherwise it's overkill for gifted kiddos.
I use it also to tutor kids who are being taught with sub-standard texts at local public school.
http://www.mathmammoth.com/books.php
