I bought 6 groups, installed them on 5 bikes, 3 broke. Always the RD, always for no reason. Mine (#4) has problems with the charging port. #5 is reporting that the battery started getting drain for no apparent reason. I've put 9k km on mine.
I like it. When it works. You can calibrate it so it works really well, but it takes work. Because it's much more flexible (it's not shimano 11s w a shimano cassette and a shimano crank type of deal), you have to put in the work. Sometimes it gets out of whack and you have to recalibrate it. If you change the cassette, you need to recalibrate. And so on.
It is quirky, for eg, bleeding the brakes requires loosening a screw that's not on Shimano, so if you bring it to the shop, they make break something or get confused.
A friend says shimano brakes are better, so it might be something you can improve by trying various pads, hoses, discs combo. Again, you're not in an ecosystem where things have been chosen for you.
Since i've put expensive batteries, i stopped worrying about having to charge it very often, and shifting is very reliable.
Now. Say you go ride the Atlas. Or go on a bike packing trip with friends. Or race. Or ride in the rain a lot. Or ride roads with lots of vibrations (one of my groups died during a downhill that had bad asphalt and a lot of road buzz). Would i recommend it? No, because if your RD dies, you're F'ed, and F'ing the people with you.
Imagine you're training all year for a triathlon, and it's on that day that the group decides to die on you, for absolutely no reason. You will have a bad day. I'd say if you're a triathlete, given how expensive entry fees have become, buy shimano or sram, be consistent
I dont race, and i bought a spare group so i can replace the RD if mine dies. And i bought a spare RD for Friend running #5, so he also has that redundancy.
We got the earlier versions, as we ordered in the summer of 2023. However, it is evident that the same issues still happen, albeit seemingly less frequently. So you would expect the groups to become more reliable, but that's not fact.
Half the reason i ride what i ride is to experiment. This is not for everyone. I got the groups for c.420 USD. Would i spend 700 for them? no. If i had to spend much more than that, i'd probably use mechanical shimano 11s 2nd hand. Would i spend 1k to get shimano di2? most probably not, i just find it excessive. Shimano and SRAM have been patent milking for years, and i dont care to finance the groupsets of pro teams.
Given how annoying it is to fully route houses, if i ran mechanical, i might run the shifting hoses externally to avoid 90 degree kinks in the cockpit, and then you should be able to get super crisp mechanical shifting.
One of my friends is back to riding his sworks because he can't get the ER9 RD to shift properly and i've been away, and he's as mechanically inclined as I am a good dancer. So i'll have to re-re-re-recalibrate it myself so it shifts well again. He likes it. When it works. He is one of the 3 whose RD died, so basically half the season, he had to use his old bike.
So, me and my friends riding the er9 have mixed feelings about it. When it works, it works. But it tends to not work, when it stops working, it's for no reason, and it requires much more baby sitting than sram or shimano.
And dont get me started on warranty claims.
Your mileage may vary.