I have used XSI a bit at my new job, and after going through a short crash course with some seriously seasoned 3d cats, I have to say, I may never look back to maya after a couple months here. There are sooo many tools in XSI that I think take the cake, smash and eat the cake, and then throw the cake up and force feed it to you like its the momma bird and you're the baby bird, and you're hungry, and the momma bird comes back and gives you the food by barfing it to you, but you don't care because you're a hungry baby bird who craves to feast on the flesh of the innocent.
And another feature that I like is the surface deformation tool. It is a deformer that allows you to take an object, and deform it by clicking on it, and then the surface you want to deform it to. For instance, you can make a drop of water slide down a leaf with it but applying the deformer, and animating the drop sliding down in Y. Capice?
And also it has default dual screen layouts built in. That is essential.
-Z