I pulled the trigger and ordered eR9. Along with many of you, I agree that $150 might not be worth the ~55g weight reduction. I can easily save that weight from other reasonable components. Including tax and Aliexpress coupon code, I got the set for ~$530, which makes things very interesting. I plan on staying on 11 speed so that I don't have to upgrade my relatively new cassette(s) that I bought for my bike and trainer.
I have to admit that the price of 105 Di2 is also very tempting. I had seen listing on Aliexpress ~$850 for an almost complete groupset (excluding rotors), which are no longer available. But accounting for shipping and tax, the overall upgrade cost for 105 Di2 is still much higher vs eR9, especially for someone like me who only needs to upgrade the shifter and derailleur combo.
I will try to give an update whenever I get them on my hands and do some testing. I am now eagerly waiting for them to arrive and install them on my bike.