The main attractions for me are it's noise, I think the exhaust note is possibly the best I've ever heard on any car, real F1 sound, how does it sound in the cabin though?
Does it feel quick or does it's mass dull the acceleration?
How has reliability being for you?
The engine note in the cabin is spot on for my tastes. You don't get an earful of exhaust, what you get is the very warm musical tone of the engine. At full throttle over about 3k rpm it sounds wonderful.
I've owned mine for almost 4 years now and it still feels very quick - you've owned some pretty quick cars so might beg to differ, but I honestly don't think you could really need anything quicker. It's so quick it's pretty much impossible to really stretch it's legs properly on the road. The good news is that unlike the F10 M5 which I found a little too "perfect" (for example stunningly fast acceleration throughout the rev range), the e60 has a bit more character and needs the engine to be worked a little harder to extract the best from it. That's not to say it doesn't pull below 3k rpm. While not known for it's torque, it's still a big engine and is flexible enough to still cruise very well when you're not gunning for it.
Reliability has been better than I'd anticipated. I have the BMW extended warranty on mine and in 4 years and ~26k miles (it's now on 67k) have only had to claim on it once (for the bluetooth module of all things). Otherwise it's just been routine servicing.