“The Casper aspect of Casper, where two conflicting blocks should not be able to be finalized, seems to actually happen running and the network agrees all along what the final chain is. So that part has been totally successful,” Vitalik Buterin, Ethereum’s co-founder, said in a research update during the developers meeting.
The Proof of Stake hybrid Casper is on testnet, and has been running for a few weeks, but not without problems. The nodes apparently keep reconnecting and randomly get a flood of connections, so devs have to perform round-about hacks to keep it going.