In pictures (but a few words, too):
Reason #1: no more cables for the derailleurs! The derailleur cable routing on both my bikes is gloriously empty. No cables means no more dealing with ‘stretching’ and adjusting the shifting all the time. Which brings me to reason #2…
Dialing in the shifting is a breeze: just press the small button on the indicator console (see picture) and the rear derailleur goes into ‘micro adjust’ mode (indicated by the red light). Use the shifting buttons to make micro adjustments until the shifting is dialed in.
Reason #3: shifting is a incredibly easy. Those may look like shift levers but they are actually just buttons. Press them like you would normal shift levers and small motors in the front and rear derailleurs do the work for you. Also, since there are no levers there is no shifting noise. This means you can be somewhat stealthy when you attack out of a group – no more clicking noises! You won’t announce your attack to the whole peloton when you drop it down three gears. (Reason #4)
Reason #5: You can shift the front derailleur under full power. So if you crest a hill and need a big gear fast just press the button and continue to hammer – no need to let up while the derailleur moves. Speaking of the derailleurs, the whole Di2 system is always aware of what gears you are in for both the front and rear. As you move through the gears with the rear derailleur, the front derailleur automatically auto-trims to prevent chain rub. That’s reason #6.
The Di2 battery. You might wonder, “What if the battery dies?” That brings me to reason #7: When the battery dies the front derailleur stops working first and you have about 50 shifts left for the rear. This should be enough to get you home. In my opinion this is better than the analogous mechanical situation where your derailleur cable snaps and slams you into the hardest gear you have.
Of course you should never be caught with a dead battery since you have this indicator console – it flashes red when your battery is about to die. With 1000 miles of riding per charge (that’s approximate) and 1.5 hours to recharge, you should really never have any power problems.
Reason #8: no more bar end shift levers on your TT bike – just sleek, aerodynamic buttons.
Reason #9 is, in my opinion, the coolest thing of all: Shift buttons on the aero brake levers. Ever get caught on a long climb in your TT bike and have to reach out to the aero bars to shift? Well that’s no longer necessary!
Reason #10: the Di2 rear derailleur. You won’t find more precise shifting on any other component group in the world. It’s just that good!