I have used NET-ENIs quite a bit with SLCs and Micrologicx. I have not had problems with communication between PLCs or even PACs (Logix platform), but often have problems with RSLinx correctly identifying the PLCs, especially in a spotty radio network. Sometimes this can be fixed with the latest version of RSLinx.
I have also found that setting the PLC communication channel to the highest baud rate speeds up the identification process. Turning the ENI off and on after making changes to the ENI also helps (this is true for any AB comm adapter). Allen Bradley will tell you that there may be problems with old revisions of PLCs, but I don’t know if this is true or just an excuse.
If you have the means, the Micrologix 1100 and 1400 are currently my favorite PLCs. They are very low cost ($500-$1400), very capable, expandable, and have a serial and ethernet connection. One of the AB tech support people I spoke with even recommended using the 1100 instead of the ENI.