I've never owned one, but on the weekend saw one in action and now want one to make working on my two cars easier. I don't know what to look for though. It's more for quickly zinging screws / bots / nuts on and off than true impact work (I've got a 500nm+ air impact wrench for that).
Name brand or cheap Ozito? Gumtree or new?
The same brand as your cordless drill so you can swap the batteries between them.