Here is what a few Experts and customers say about Dura Ace / Ultegra Di2 Road Bikes:

VELO-NEWS SAYS: BEST-SHIFTING-EVER, EXTREMELY DURABLE

"…eight months and thousands of miles later… Di2 offers the best front shifting available, bar none."
"...A winter’s worth of wear and tear hasn’t done anything to degrade the performance.."
Read more at VeloNews

CYCLING WEEKLY UK SAYS:
" Di2 will measurably improve riding" "(No) reason why you would choose a mechanical option...really is that good"

ENHANCED CONFIDENCE AND CONTROL"Electronic shifting…now as easy as tapping a keyboard. The smaller hood radius and adjustable reach has banished any problems…fingers confidently wrap round the brake levers by at least two knuckles, both on hoods and in drop positions, giving me absolute certainty in my stopping power." "…new found confidence…knowing that I’m in total control no matter what my position. There is no way I am alone in feeling this way as a woman with regard shifting and braking. Ultegra Di2 will measurably improve riding…"

INCREDIBLE BATTERY LIFE
"How long does a battery last? Months. Even pros get by with four or five charges a year. The reality is, twice or three times a year you’ll need to top up, and you get plenty of warning when the battery falls below 50 per cent"
What Happens When Battery Dies?
"The front derailleur uses far more power, and stops working first...plenty of power left to get you home and be able to use the rear the whole time. If you end up on the road with no power, you’ll have ignored a LOT of warnings."

EXTREME DURABILITY
"After a winter of salt, grit, road spray and general abuse, how unaffected the performance and well sealed the connection were also impressed immensely. A few forays onto cyclo-cross territory were also impressive and very well suited to the precision. Being completely unaffected by dirt ingress was a revelation to hardened racers…" "...we can’t see any reason why you would choose a mechanical option. Yes, it really is that good."

Will it short in the rain?
" …through terrible winter weather…never had any issue with water ingress. Pro cyclo-cross riders use Dura Ace/ Ultegra Di2…their bikes get jet-washed during races."