My character is an 11th-level druid (about 500 XP short of level 12), and he only has animal companion(s) for a short while... He had a snake he used mostly as a decoration, wrapped around his neck -- it was a 2-headed viper from FRCS -- which wasn't my original intent, but snakes are rather weak creatures to begin with, plus they have Int of 1, meaning only 3 base tricks... not much to play with. Then, for a short while, I tried having a dire bat... didn't work out, really. Preparing for combat took almost twice as many spells (greater magic fang, barkskin, nature's favour, etc...). In the end, I figured I'm better off without a companion.
In our other campaign, another player plays a druid. She's phyisically very weak, and depends on her wolf to protects her. That party also lacks a dediated fighter -- the closes thing we have is an archery-oriented ranger, so the wolf plays a pretty big role.