I can scale PostgresSQL pretty well, thanks. PostgreSQL is used on some of the biggest databases in the world, but no one's going to tell you which. :)

Your benchmark is pretty much irrelevant because Clustrix sells hardware appliances, not the software standalone.

Edit: That's not to say MongoDB didn't fail your benchmark. But the issues you highlight (single mutex) are known. Test against a real database and your results don't look that impressive for a 10 node setup.

All that said, Clustrix looks great. Just make it available sans-hardware.