I think it will depend on the release.

Adoption of 0.10.0 is looking really good: https://getaddr.bitnodes.io/dashboard/?days=90#user-agents
... going from about 3% of nodes to 12% in just the last three days.

Different versions existing on the network isn't a problem until there is some incompatible change in either the consensus code (a hard fork) or the p2p networking protocol (and even that doesn't have to be a problem if there are "bridge" nodes that speak both versions of the protocol and relay blocks/transactions across the incompatible networks).