I really, really liked my E39 M5. That said, I love this car too. Do I miss the old one? Sure! Imola Red on full extended Caramel leather was a sight to behold, and I got compliments on that car weekly. Why did I get rid of it?
1) I owned it for five years and was ready for something new.
2) The styling, although classic, was beginning to feel a little "long in the tooth" to me.
3) I am a power junkie and the thought of 500hp was enticing. I was NOT disappointed.
4) I am a tech junkie, so having things like SMG, HUD, M button, Active Seats, Integrated iDrive system with iPod, Sirius, etc..., EDC Sport Suspension, comfort access that unlocks the doors for you and wont let you lock your keys in the car or trunk, ...the list goes on and on, was very enticing. Again, I was NOT disappointed. SMG may not be for everyone, but all you need to do is put every setting as aggressive as you can, punch the throttle full bore and feel a shift at 8,250 RPM's to know if you love it. I have only done it a hand full of times, but man it is awesome!
In summary, I love both cars for what they are. I am very glad I owned a virtually trouble free E39 M5. My experience to date with the E60 M5 has also been awesome.
The reality is with MORE technology, gadgets and power comes MORE maintenance and more things that can break. I guess if you want to play with the big boys, you're gonna have to pay for it. I'm willing to pay, because this car rocks.