Never leave home without them. I'm serious. This isn't one of the best Circle units. This is one of the best units in the game. Period. :)
This was the first unit I bought and painted and as soon as I started playing I realised they're literally insanely good. So far I don't have doubles of any units, but as soon as I've finished my collection of Tharn ladies, I'll get another unit of Druids.
The Overseer is really a great unit attachment for the Druids. I've met Menoth lists who couldn't touch them because of the Elemental Immunity spell and they can be about as annoying for Legion. The Devouring is a great spell with decent POW and the Overseer will increase the Counter Magic radius. However, if you can't find room for him in a list, the Druids will do fine anyway.