As a creature, it cannot attack in the same turn it was cast.
As an artifact, it can be tapped for mana in the same turn it was played.
When a non-creature becomes a creature, it follows all the usual rules for creatures. What matters is whether you've controlled it continuously since the beginning of your most recent turn (including the current one). Waking up a Keyrune doesn't cause it to leave your control or become a new object, so as long as it's been on the battlefield long enough, you're fine.
Just a note: if you animate a Keyrune and it still has summoning sickness, you won't be able to tap it for mana.
I knew it! Me and a friend were arguing about this when the new sets came out, I said they could attack, they said they couldn't.
If it had summoning sickness when it was transformed, then the ability of Izzet Keyrune would be useless because a blocking creature won't deal combat damage to a player.