The rattles come in over time, as for how annoying depends on how sensitive your hearing is. For me I could hear them over the MM stereo while set at volume 12 and on the motorway. I suppose they aren't that annoying, and you do get used to them, some IS's I've driven have been worse than what mine was others slightly better but none silient.
All the new style IS's I've driven have had the problem.
Mine has now been in 4 times spread over 15 days to get it resolved and now it's 95% sorted once they actually listened to me telling them where it was rattling.
Has to go back to get the front door rattle done then it'll hopefully be quiet appart from the dashbaord. I didn't let them work on the dash as since the car is only a few months old I didn't want it made worse and it's only every so often.
Only downside to getting it fixed is that my roof has a lot of oily marks over it and they still haven't cleaned it properly. Still when it goes back hopefully it'll get valeted properly then. Apart from that I am extremly happy it's almost quiet now.