IS is a good car. G35 sedan isn't a bad car either. e46 is a great car. The newer A4's are nice too.
I found the e46 sedans the most fun to drive, BMW just has the handling done so well from the factory.

The interior is still great, I like the e46 interior more than the e90 for a variety of reasons. You could get a 320i and it's still fun to drive.