I've found those (and similar) good too, but I'm talking about a 6" waste pipe in a hole with more pipes inside it.
A normal saw would have been fine if I wasn't trying to look after the contents of the pipe. As it was I got 90% of the way round and kicked it a few times, which then broke the end off at a funny angle.
We use waste pipe as ducting because it's cheap - we get huge discounts at Jewsons on it, up to 90% on the fittings.