ER-X is a solid device. I've had one since February and it's been great.
I set mine up using the wizard
Only tip I have is to perform the code updates via CLI. I have mixed success with using the GUI.
I added a UAP-AC-LR in May, it's solid as well.
I use the POE adapter from the WAP to power the ER-X, and port 4 passes thru POE to run the WAP
So far, no complaints