We're forced to regularly ban Mibbit users on Freenode technical channels due to abuse masked by Mibbit's proxying, and for that reason I must say I'm not a fan.
That's the double edge that comes from any ease of access. Mibbit makes IRC very simple and very easy, and so more people use it, and with people come trolls.
That said, if you're looking to access IRC easily, that's the way to go.
It would be quite easy for axod to pass along user's IP to the server instead of masking it through Mibbit. Having the user's IP as their realname isn't a real fix..