I just got back from test driving a 2005 530i and 545i. I liked them, but was not that impressed, especially since the 545i has 325 HP and good performance numbers. It just didn't feel that fast and seemed a little loose in the turns and was surprised is was not that fun to drive. Now it didn't have the sport option which gives it better wheels and suspension, but wasn't anywhere near what I expected it would be. The idrive is interesting and seems a little confusing/distracting when driving, but I could probably get the hang of it.
I really think I want a used M5 after driving the 545i, but then again I haven't driven the M5 yet and there aren't many in St. Louis. Those damn repairs make me nervous though, so I'll have to get a CPO vehicle. I really hope the M5 is a lot more fun to drive because my NSX was a million times more fun than the 545.
It makes me want to keep what I have.