How well Checkmk scales
A large food retailer in Germany uses Checkmk to monitor its infrastructure. This is their IT monitoring:
stores connected nationwide
checks executed per day
The whole Checkmk environment has a very mature, stable foundation of supported checks and performance counters. Yet it grows almost on a daily basis.
Support for new systems is being implemented very often and if you are willing to request it yourself, you can have it included in the core distribution in very little time. This agility really has no match among the larger competitors.