Serious answer, about Di2. It shifts really well, probably not better than a well tuned DA mech system, but not worse either. Unlike mech, it never needs adjustment. Your Garmin can show you where you are on the cassette so you don't have to glance down, other cool integration stuff too.
If I went back to mech the biggest annoyance would be the trim. (Don't tell me about yaw FDs, those are crap.)