They are pretty accurate if they have a little experience, especially considering that they are generally being aimed at a large mass of troops. Trained in a province with +3 or more experience bonus, they are quite the handy tool indeed.

I have had one Onager in a single unit destroyed by enemy Onagers. I quickly destroyed the enemy Onagers before they were able to destroy another of mine. :)

3 Onager units with 3 Archer Auxilia can rout an enemy army before they even have a chance to reach your lines. Don't forget to bring along some cavalry to mop up the whole mess at the end.