I'm very interested in experimenting with na-ion this summer. Their specific power/energy densities are still low compared to li-ion but their cells are already available for cheaper than li-ion despite having not reached economies of scale yet. They also seem to be reporting (unverified in data sheets) longer cycle lifespans and larger operational temperature ranges. Could make for a very nice self managing solar powered pi project soon.
If costs drop fast, the lower density won't even matter- it could be the kick in the ass for utility scale battery storage to take off.