I looked at an 2005 M5 when I was buying. I wasn't really impressed with it. I didn't think it was very refined. I was also worried about the reliability of it, but it is a great car. Obviously I chose the F and have no regrets.
The more time passes since I chose the ISF, the more I feel like I made the right decision.. It was like I was trying to convince myself that the M5 would be reliable, but simply couldn't. There were too many stories of random issues that would shut the car down: Clutches, SMG in general, seals on the SMG, steering angle sensor in the column, Diff's not liking to be flogged, etc. And on top of that the stories of warranty denials..