Jezzer,
I'd be very interested to know how the IS compares to the 5 series. Which 5 series have you got ? The E39 or E90 ?
Which engine ? If you have owned a 5 series and driven (and liked) the IS then it must be a very good car.
Errr... actually I had an E60 - the current 5-series. Nice car (I had a 525d auto), quick, horrible run flats (get the pressure wrong and they tramline like mad on motorways) infuriating iDrive (apart from the fact it just talked to my phone, got the phone book with no fussing which is better than the MM system). I'm currently getting better mpg out of a 250 (believe that if you will!) and I have a sound system (the Mark Levinson system) that just blows it away. Incidentally, the ML system is much better than a Porsche Bose system too :)