On the v8 645/650 watch out for white smoke at idle. These engines are known for burning valves stem seals. Have a look over on the e63 Series sub-forum on bimmerfest, some great advice to be had over there.
I have the baby and the daddy of the range, I have the 630i (with SMG) since May 2010 and it has been reliable in that time needing only a battery, a few suspension components, an electric water pump, rocker cover gasket and the usual service items. I have the M6 since February of this year and it was a rocky first few months of ownership but it has finally settled down.
I think they are holding their age well. SMG is not for everyone, both mine have it so it is "normal" for me, I have driven the auto and it was ok for the limited the me I had it, never driven the manual equivalent. They have a lot of parts in common with the e60 5 Series.