On the left is an icon for an ethereum address that I use constantly. @austingriffith built an identicon miner and I requested him to try to spoof it. This is the result after only 45 minutes. Imagine if he had left it running for a day or more! (addresses completely different)
When I was working on EthAvatar (https://github.com/ethereum/EIPs/issues/928) I was worried about an attacker replicating an Ethereum identicon (Blockie). I wanted to test how easy it would be to brute force replicate a similar enough blockie you could use it to phish an account. My conclusion was you could get close, but it really isn't worth it.
Blockies are also very important in my game Galleass. Each ship has a Blockie flag that represents the account countrolling it. I would love to find an account that looks like a "jolly roger". Here is the Boba Fett helmet I mined controlling a ship in Galleass: