I've heard too many expert opinions on the matter to be able to decide the truth. One issue, is the lack of control over what ingredients go into ABS waste pipe. It may contain a number of things, including heavy metals, and other unknown recycled plastics.
Some claim it has an anti fungal/bacterial agent in it to avoid build-up on the pipes interior, while others claim it doesn't. It will become inert once covered in bio-films (slime), but I choose not to use it anyway. It isn't certifiable for potable (drinkable) water due to the contaminants that are deemed acceptable for waste drain pipe standards. If it isn't deemed safe for me to drink out of, I won't let my fish drink out of it.
It is definitely not as strong as schedule 40 PVC and it breaks down when used for UV sterilizers. Schedule 40 PVC is generally cheaper and has far more options for fittings.
The ABS plastic used for aquarium parts is of a different nature and has not been reported to contain anti-fungal/bacterial agents, or anti-clogging chemicals (stuff that makes it slippery).
You'll get a whole mess of "I've used it for years" anecdotes, but you'll hear the same reports from people that eat fatty foods, drink heavily or smoke. The problem is you don't know what the negative aspects of using it are. It may cause a little more algae, or it may interfere with endocrine development. It certainly won't cause any obvious problems.
PVC and ABS are both carcinogens, so you just have to choose your poison.
