I use Inventorcam where I work, and most of my problems come from not knowing how to draw in Inventor. If the Geomertery is not there, I have a tough time drawing it. Other than that, I would say it's a pretty desent package. Although I found out it's not as automatic and intergraded as they say it is.
I've used Gibbs in the past, and if it wasn't for the money thing, I would perfer to use that instead.