Laurence Tratt from King's College London has written a long and detailed introduction to the goals and significance of RPython over on his blog. Laurie has been implementing his Converge Language in RPython in the last months. He is one of the first people external to the PyPy team who have pushed a sizeable RPython-based VM quite far, adding and tuning JIT hints. The post describes some of that work and his impressions of RPython and PyPy.
"RPython, to my mind, is an astonishing project. It has, almost single-handedly, opened up an entirely new approach to VM implementation. As my experience shows, creating a decent RPython VM is not a huge amount of work (despite some frustrations). In short: never again do new languages need come with unusably slow VMs. That the the PyPy / RPython team have shown that these ideas scale up to a fast implementation of a large, real-world language (Python) is another feather in their cap."