I think it's pretty clever. The memory fragmentation caused by incessant consing leads to gc phase after gc phase. Their ORO trick prevents this, and as a bonus improves cache locality. They report they also have a true mark and sweep phase to take care of the corner cases.
Does their ORO scheme have any significant downsides?
No sharing. And clever compilers can already allocate most conses on the stack nowadays.
OSNews has a detailed article that takes a deeper look into this new dialect of Lisp.