Anyone got any experience/feedback regarding large scale Zabbix monitoring?

I'm currently embarking on a project to implement a distributed monitoring solution across 40 sites globally. The goal is to have visibility of up/down/broadcast/multicast traffic at port level for troubleshooting and trend analysis. The number of (Cisco) devices to be monitored is between 12 and 30 per site with anything up to 300+ ports per device. Currently looking at Zabbix.

Anyone have any advice/pointers/other recommended (preferably open source) solutions.

From a tech/user perspective I need to be able to present a site level dashboard giving an at a glance view of network health in addition to drill down to port level.

Thanks in advance for any advice.