I've got Di2 on a bike, and it's fine. Doesn't really shift any faster or more precisely than a well-maintained mechanical groupset, but it does stay in adjustment more reliably. And with the dongle for wireless connectivity, there are some pretty useful features with integrated cycle computers and such.
Battery life is not an issue for me: even with lots of shifting (I'm in a very hilly area), it runs maybe 100 miles per 10% of charge.