Yep Blur studio are awesome, and incidently that 'Gentlemen's Agreement' short someone posted here a while back was done in XSI by them. See this interview for some interesting thoughts on XSI:

Interview with Diego Garcia

Quote - **
What were the most challenging aspects of your work for this project?**

Blur has been traditionally a 3dsMax house but, at some point two years ago, we realized that we were reaching the limits of this software with our rigging pipeline. We started to look for other solutions and it turned out that XSI suited our needs perfectly. We also could count with the help and support of SoftImage Special projects which happens to be just around the corner from the studio.
Gentleman's Duel was the perfect project to start the transition to XSI, so the most challenging aspect for the tools team was precisely changing the whole Animation and Rigging pipeline to XSI. We had around 6 months to make it happen and production would be happening at the same time we were working on it... Also, for all of us it was the first time we programmed tools for XSI and we had to make sure animators and riggers would have the same set of tools that they were used to before when they were working in 3dsmax.

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What do you think are strong points of XSI?

In general XSI feels more consistent than other packages, it almost never crashes, it is efficient with large amount of data, modular, and everything works with everything, a new feature is the same well integrated than the old ones. References is another strong point of this package, with the new delta referencing in XSI 6.0 it will allow pipeline developers to overlap the processes even more, with the amount of time and money saved that represents. And for SoftImage as company, it is just the fresh ideas that they have ( Face Robot, GATOR, MOTOR ) , they are willing to listen and help their users, and that they are open to work with different packages like 3dsmax or Maya.

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Which areas should be improved?

One of the areas I haven't gotten used to is the UI programming, It seems to me that is a bit limited, everything has to be gray or you have to develop some kind of hybrid application using the netview. That is why we have taken a look at other alternatives like QT or pyQT. It would be nice if it were possible to create your own native UI controls, treeviews, spinners, etc ... Also, since we use it as our main scripting language, I'd like to see more integration between python and XSI, for example, at the moment we develop our custom operators in Javascript or C++ because of evaluation efficiency.
From the artist point of view, I think it is well known that XSI lacks a robust particle system ( although from what they say on their website it seems they are working on it )