Feature rich, extremely powerful, support for a wide variety of vendor products and technologies. Simulates many technologies to a high degree of detail. Integrates with VNES and traffic captures from ARX as well as other generic traffic files.
Room for Improvement:
It can be difficult to use, not because the UI is bad, but because it does so much that there are a lot of controls. Scaling - You can't distribute the load, you'll need a lot of processor and RAM for large tasks. Learning curve - but they offer free training.
There hasn't been anything as a network engineer that I haven't been able to simulate with this tool. It even has a python interface to script batch operations. Failure/survivability analysis, routing redesign across the entire network, traffic engineering, queuing optimization, etc. It can be used for troubleshooting, but I find the traditional methods more effective. The ROI come from the unique ability to perform design actions in a simulated environment. This saves time in the dev environment and provides the ability to work through design issues using the entire simulated network. For example: The network was designed using OSPF with one area, it grew and later you want to redesign the areas. This can be accomplished best using the model as a design tool.