First let me say I've had my 2004 545 for almost a year now and have no problems other than a rattle in the passenger seat that was promtly fixed. I love the car, drives like a dream. The problem with the new E60 is that BMW, rather than focusing on their area of expertise, driving mechanics, has tried too quickly to jump into the same "luxury" market that is dominated by mostly the Japanese with all their high-tech electronics stuff. The Germans have never been very good at that stuff and they should stick with what they know. I am intentionally ignoring the exterior design issue because we have all seen the threads and everyone has their own opinion. I have mine and put my money where my mouth is.
Most of the problems on the E60 have been cars with the electronic bells and whistles like Voice Command, Navigation, Active Steering and Blue Tooth telephones. The key is does car have the Nav system, which uses a different computer than non-Nav cars. Most of the lemons/problem cars have this CCC computer vs. the MAS-K computer used in non-Nav cars. My 545 does not have NAV or Active Steering or any of that other crap and I've totally enjoyed mt first experience as a BMW owner.
IMO, you want all the toys, buy the Acura/Infinti/Lexus. You want a car that drives like nothing else, get the BMW. But BMW's have always had a reputation for being a bit finicky in terms of maintenance. The electronics problems have greatly magnified this for BMW, and of course the technicians at the local dealer are not being trained very well either. Where BMW blew it was trying to catch up to the Japanese tto fast.