Do you currently have any shifting problems? If not, then I'd wait and get it when you get your next bike. My 105 5800 I put on my old Paramount shifted flawlessly, immediately and quietly in all gears. Only needing trim in the big big combo.

I just got a new bike with Ultegra Di2 and I see that I will eventually get to where I wouldn't want another without it, but....... I wouldn't spend the extra money to upgrade. So far my 105 bike is quieter, shifting is the same. I only got the Di2 bike because it wasn't much more during the spring sale than the same model without.

I too would rather save the money for a power meter and some other things.