Hey Philip,
I was over on the Z4 forum myself for a while. I also wrote a column for Performance BMW magazine for several months.
The ISF is an incredible car in every sense. The ride is considered by some to be too firm, but I don't think it's very firm at all! The E85 3.0i Z4 I had was firmer in my honest opinion.
It's all relative I guess. with a rolling start, coming off a roundabout, I slaughtered a Porsche Carrera 4S the other week! ;)
There honestly aren't many things that can show up an IS-F. Servicing costs are a lot better than the M3 as well.
You also have only 200 or so IS-F's on the road in the UK, making it very unlikely you'll ever see another one! :)
Don't mind Dave, he's a Northerner! lol. ;)