I haven't yet heard any voice synthesis that isn't buried deep in the uncanny valley. It's exceedingly creepy. A huge number of Amazon Prime educational/documentary videos use synthetic voices that are good enough that I have to listen for a couple minutes to be sure they're fake, but the whole time it's unnerving. I have no doubt that they'll nail it soon (and maybe Replica already has).
Despite any misgivings, I really hope David Attenborough gives permission to replicate his voice in perpetuity. I want to hear him narrating Planet Earth forever.