The 5 series has been around for a long time, but if you are looking at the new 5's, then you are looking at a different kettle of fish. The 5's are generally around for 7 years - so for example, 97 to 2003, with the new model out in 2004. Some feel it is better to wait one year and get the 2005, or in the older series the 1998 versus the 1997 model. Some of the bugs will have been worked out. If you followed this philosophy out to its logical conclusion, the very best model to buy would be the 2003 as it would have most of the bugs worked out of the 97 - 03 line.
More will go wrong with a 5 series car than with many cars because the 5 series has so much more in it than most cars - so the logic is simple - more in it, more to go wrong. However, if you ask most of the 5 owners, they will tell you they love their cars.
Now the new 5 series is filled with controversy as you would expect a new line to engender. But this new 5 has strayed from the "traditional" looks of a Bimmer and it has idrive, but fortunately a dummed down version of the 7 series idrive which was difficult to manage for many. Idrive is managing your car via the computer which is a logical progression for cars to evolve to and BMW is the first to do it - of course. But idrive isn't totally user friendly yet, though the 5 series is better than the 7 series.
So you have two crowds - the traditionalists who would probably purchase a 2003 model and the newbies to BMW who would buy a 2004 5 series not realizing the controversy swirling around the new 5's.
Where do I stand? Well I just bought a 97 528i which I plan to own for about a year and a half. I will trade it in for a 2003 5're which I will own until the 2011 model comes out.