Here's a KB Article from VMware on how to setup SNMP on ESX/ESXi.
VMware KB: Configuring SNMP Traps in ESXi/ESX
We also use SNMP with Solarwinds Orion for monitoring. Orion uses SNMP to poll the hosts. All we do is enable SNMP and set the community name. This enables our Monitoring engineer to have any of the orion nodes poll the ESXi hosts.
However, we're having an issue with ESXi 5.5 where randomly, SNMP stops responding. Randomly, a short while later, it will start again. I am working with VMware support on the issue but have no resolution as yet. I'm running ESXi 5.5 U2. Didn't have these issues on my ESXi 4.1 hosts. 