I've been waiting for this for some time, and am happy to report as of today or so, portions of Art of Problem Solving's new Elementary curriculum ("Beast Academy") are now available. I ordered it. It's shipping Media Mail, so it'll be awhile before it gets here but I'll report back after it arrives and we take it for a test drive.
So far, they've released 3A and 3B, obviously part of their third grade curriculum, though they're working on grades 2-5. We're currently nearing the end of Singapore 2A (and halfway through Math U See Beta), and though the 3B materials look a bit beyond my oldest yet, 3A is within close striking distance, based on the assessment test.
The samples look fun and my daughter was stoked when she perused the available pdf's.