What do you mean by large and complex? Size of a European country? every network technology from DSL through 100Gbps DWDM? couple of thousand switches? couple of hundred routers? VRFS? lots of IPv6?
Answer these questions:
1. Can it manage everyhting we have?
2. Can it manage everything the way we have it configured (or do we need to change that)?
3. How will we scale it to meet out current and future needs?
4. Who will stop me from buying this?
5. Would I bet my job on this being the solution?
/RjL