I had an isF before I picked up my e93 m3. The isF def feels faster IMO but thats probably because of the difference in the amount of torque between both cars. And yes, I have test driven the e92 as well.
I loved my isF. The interior was gorgeous, much more luxurious than the m3, but not as sporty. Exterior of the isF is subjective but there were definitely some angles which made the car look very sexy and others where it looked..well less than that...
Both cars are GREAT. Yeah the m3 might be better in some or most areas but the isF is not that far behind. It was a definite joy to own and unless you have driven one yourself, I don't think its fair to bash on it