My experience with mechanical and Di2 Ultegra is that the shift quality is pretty close to the same. The electronic one just never needs to be adjusted, which is nice, but not a huge deal. I prefer the tactile feel of mech, it's more satisfying and I feel more connected to the bike.
Honestly the best thing about Di2 is auto trim.
Maybe it's just me but I like to tell myself everybody lets the battery die once while riding. You learn your lesson and never do it again. You try to move into the other chainring and nothing happens. You try it again, and again nothing, then it hits you. Now you've got two or three dozen (?) rear shifts so use them wisely. It's not a big deal to charge, but it only needs to happen every few months so it's easy to lose track. I hate the flashing light thing, very glad my Garmin can tell me what's in the Di2 battery.
Like almost everybody else, I would want electronic on any future bike. The perfect shifting every time and auto trim converted me.