Good post regarding the drawbacks of taking onagers along when operating in the 'forest lands'. And yes, no matter where you're going - having onagers definitely handicaps your rate of movement.

Still, there are times when I like to have them along - especially on the rolling hills and open plains. Coupled with archers (another one of my favorite units Mightilyoats), I like to peck away and harass the enemy before they get to my main line.

I saw one post (in the guide section) where a player likes to have six (6) onagers in his primary assault army. Talk about 'cry havoc'! That would soften up the line a bit.