Fusion is like Inventor LT but from 5 years into the future.
It also uses a tree, but the tree is infinitely more powerful... Fusion works considerably differently, much slicker and more powerful in a lot of ways than Inventor, like something from the future, and Inventor is continually taking on more and more aspects of Fusion like bits of UI & now the modeling engine... but Fusion is nowhere as complete as Inv yet, lacking some of the more intensive features, like Lofting, Parametrics, & FEA & whatnot.
I installed it, found there's not a lot we design that we can use it for right this moment, but I'm leaving it installed, and will keep a close eye on future updates with great interest.
The prospect of tying in a rapid prototyping service or directory of providers would be huge for small firms & independents.