avids sqeal like a banshee. this seems to be due to brake arm flex. they're heavily machined at the back of the arms and, if you look down while clamping on, you can actually see a bit of flex. this is not as noticeable on a really stiff steel or alu fork, but put 'em on a carbon fork and your bike sounds like a cat in heat.
there's not much you can do about it. radical toe-in helps for a while, but you go through pads like crazy.
they're fussy to set up and i find that they don't hold up well to the rigours of racing. the little hook at the end of the brake arm that holds the straddle cable tends to crack and break. all cantis are prone to this to some extent -- though the avids seem a bit more so -- but at the price you pay for them, you'd expect them to be a bit tougher.