A few things jumped out at me
Cons:
1) have to tend to the fire feeding it small sticks for hours.
2) splitting fire wood much smaller (although less total amount)
3) 1/10th the wood seems like a pretty generous estimate in WA where we have to have the newer more efficient reburning wood stoves.
4) big foot print of the rocket mass, people are making benches they won't sit on, it takes up floor space.
5) barrel, ugly.
6) no visible flames to sit in front of and go in a trance while holding your favorite alcoholic drink
7) trickey to get started, although a boost fan would mitigate that
Pro's
1) no chimney fires
2) holds heat longer
3) less wood to stack
4) could be cool if you're an artist (I'm not)
I love the greenhouse idea, you're going to be in there pretty much every day anyways.
For my greenhouse idea I want to put in a boiler and keep thermal mass under the greenhouse in a big insulated tank. Then I can scavenge the heat off that and heat the main house with forced air. In the summer I can do the reverse to keep things cool. A modified rocket mass would be cool to heat the water with.