You're correct about the origin of nmon. It isn't an officially supported IBM product and you use it at your own risk.

A great way to monitor network statistics is on the network equipment itself via a tool like MRTG.

http://www.mrtg.org/

This gives accurate results and makes all the eye candy charts you could ever want. 

Depends on what you want. If what you are looking for is the amount of traffic that is going over a specific interface, you can use netstat to get the raw numbers for packets sent and compare it over time.

A better solution is nmon. It is a free performance monitoring tool from IBM (sort of the swiss army knife of monitoring utilities). There is a link to it somewhere on the rootvg site.

The topas utility might also give I/O rates per interface (I don't remember if it breaks it down that way).

If on the other hand you are looking for how well tuned the interface is, you can use one of the many network benchmark tools. A simple one is ttcp. Another one is IOBench.