It's noisy and the autofocus struggles in low light. Is candlelight asking too much?
But, this lens is on my 20D much more than my 17-40 and 50 as it makes a (relatively) light weight package. My 20D is my P&S (I couldn't cope with the shutter lag of normal P&S cameras) and usually I don't want to lug around a zoom when it's a "just in case" scenario.
Oh yes, and it takes a great picture too..