I tried DI2 when it first came out and I wasn't impressed enough to replace it after a crash that broke the RD and shifters. This was back when the RD alone was $400 and the shifters were another heap of cash that I couldn't justify. My wires also worked their way out of their ports on super rough roads, which I'm sure has been addressed in current generations. I'm not a fan of having to bolt/ziptie stuff like junction boxes or errant wires to my bike. I also hated having one more thing that needed charging every so often. I'm sure someone will pipe in with a "those aren't problems/hasn't happened to me" post, but it's hard to shake my past experiences on a machine that I have to trust implicitly to get me home/to the finish line.
Given my sorted history with DI2, the bike that's being built for me right now is going to be mechanical Ultegra with hydraulic disc brakes in order to get the better wheels.