EthanW -
The above replies are all true, but miss a subtle point I will now direct your attention to.
There are at least 3, types of "simulation"software.
1. "Solid Model Simulation With Fixtures In the CAD system" - I have no idea if Fushion has this, but it could well be what they'll be offering. The key point is that you put things like fixtures in the model, and it finds the places where you crash into the fixture. This is very worthwhile.
2. CAM based machine modeling - this is (or was when I bought it) a several thousand dollar option - and for at least some work, worth many times that. For this to work, a good model of your particular machine is required. These modules are often included in CAM, and so far as I know all of them are driven by the internal CAM represenation - they are NOT backplotters. But very useful.
3. Full on backplotter simulators - vericut probaby the most famous, but ther are others now. Serious money. And of course only as good as the model for the machine you actually have, and modeling of how the controller actually behaves.
In my experience #1 finds a LOT of problems. #2 finds a few obscure problems directly, but study of the simulation reveals other issues.
4. Some machines (DMGs with Heidenhain controllers among others) have code in the controller to prevent damaging the machine. They'll keep you from drilling a hole in the side of the cabinet with a DMU. But the one I have doesn't know about fixtures, so it won't keep you from slamming the probe body into a vise. (You don't have to ask me how I know this....)