Be wary of the low ink warning system - it must make Canon a fortune in our throwing away carts with up to 30% ink still remaining.
That comment annoys me. Canon have used Clear ink tanks so you can see EXACTLY how much ink is left - true it is a bit premature but anyone throwing away a 30% full cartridge when its made of clear plastic is a fool. Just look at the piccys here - click here
I find the warning on my S630 (same cartridges) very handy as it gives me a week to 10 days notice to get some more cartridges ordered.