Terradons are quite useful units. If the enemy has the flank of a gunline within 17" of a forest, then park up in the forest (for a -2 to hit with shooting for being in soft cover and skirmishers) and charge that exposed flank (and don't forget, Terradons get stomp attacks). If it goes well you can overrun into the next unit in the gunline... War machines can also be engaged in close combat by the second turn and the crew will be lucky to survive, and even if they do the machine won't be able to shoot as long as it is in close combat.
They just seem to be a bit more flexible than Chameleon Skinks imo. The rocks also do a nice bit of damage if you can get them all dropped. I'm not denying that chameleons aren't useful (I take a unit of each in a 2.5k game), just that Terradons are just as effective imo.
So my normal game plan is
12" vanguard move
Turn 1: Fly over a unit, drop rocks and land (in a forest if possible) facing gunlines flank/warmachine unit
Turn 2: Charge gunline in flank/Warmachine and destroy it.