I personally find them VERY irritating. The first time, it may be an amusing scare, but after that, it's nothing but a nuiscance. Not to mention it keeps me from clicking on advertisements out of disgust. I'd prefer it if they had the sound off by default for their advertisement (as I've seen on plenty), or if the sound were activated on a click, rather than a mouseover. And for me, only once have I gotten something other than "OH MY GAWD NOOO WAAAAY".