| Melissa Litwin |
My ranger hatched an egg and got an animal companion out of it. We waited about a year of in-game time until it was big enough to go adventuring (medium, the base size of a roc companion), then when the ranger hit 10th level the roc became a large mobile bombardment platform. It was a lot of fun and didn't really change much in terms of power (fly 80 ft is really good, but at high levels flight should pretty much be available anyway and the roc didn't have great saves).
I wouldn't let them make armies riding rocs though. There simply aren't enough eggs. The PCs probably retrieved half a dozen eggs at most, and rocs do take a significant amount of time to mature and then breed (not to mention the inbreeding concerns). Let them play with one roc as an animal companion or trained castle/city guard, but not have an army.