We have the X5M for my wife to drive, and the back seat is spacious compared to the E60 rear bench. We have three car seats across in the X5M too (two are still rear facing even) - that would be somewhat more difficult in the E60.
With that said I have a CTS-V for myself and face similar problems to what you would face with kids in the back of an E60, and I make it work. If car seats are a concern, there are narrow seats from Diono that work well. It also gives me the opportunity to torch M5s in a station wagon with little hands waving from the back seat.
