I'm referring to either front or rear and in any gear. On mechanical systems, you can feel exactly where you are in the shifting process so you can make the shift occur precisely when you want to. With Di2, you press the button and there is a slight delay while the servo moves the derailleur. It's only a fraction of a second but it is noticeable and you can tell the difference. It's a different kind of ride and you can't feel your connection with the process like you can with mechanical. For this reason, I prefer mechanical even if Di2 is faster (particularly for shifting to bigger rings/cogs) if you measure the shifting process from start to finish.

I know a lot of people shift anywhere in the pedal stroke, but I think slamming thin chains under load is asking for trouble even if it works most of the time -- particularly when switching rings. So I always shift so the actual transition takes place as my feet hit the 6 and 12 o'clock positions. This means starting the shift a bit earlier and when/how you pull the levers is a function of what gear you're in and what gear you want to get to. I know the electronic system is less prone to user error, but I don't have problems shifting or finding optimal gears.