Recent Road & Track article about the different type of start/stop technology/systems. Included some subjective ratings by the editors and IIRC, they gave BMW's either last and next to last as far as being seamless and working well. They basically said the BMW system is the least technologically advanced and not well integrated.
2009 335i e92
2013 Porsche Boxster S
2006 BMW Z4 3.0si- Gone, but not forgotten