What Michael says is true. When I found grasshopper I was very exited because of the things people do with it, but its because it is good doing repetitive and progressive tasks and paneling; that makes a model look very complex. Like Michael said it needs a lot of planning and it is useful when you are going to try different things around one model, and then before you start you plan which things will be manipulable and base your definition on that.

I don't know the other programs people mentioned but If you think parametric modeling is for you I think GH is a good solution because it works in rhino and you can just copy paste things from MoI and vice versa. GH has a lot of great things inside for making complex models and for playing around in the design phase and to mention a couple more features it is easy to use compared to other software and it has a great community almost as good as this one.

Currently I'm working on a model that has a lot of parallel planes that morph from one form to another. That can be done with a sectioned loft but I'm using GH because: Im not sure what the final form will be nor i know what the final spacing between planes will be because its not constant. Using a traditional approach it can take a lot of time and I can get tired of making a new model each time i think of a change. Here GH become useful. Any way I always draw in MoI the things I need and then copy them to rhino, and in previous works, after making all I want in GH i bring the model to MoI for the final touches. In complex models MoI and GH make a good team.