The success of this will depend entirely on how it's priced -- if they make it reasonably affordable (read, in-line with the current Mac Pros) it might do pretty well; if they price it any more than that it'll be the new Cube: an interesting lark but not really worth the cost ...

Mainly, I'm wondering if the graphics cards will be replaceable --- that technology seems to advance so rapidly that I could see a need to replace those before needing to buy an entirely new machine ...

But in my line of work, the ridiculously small enclosure + graphics and processing oomph look really attractive. We'll see if Apple prices it out of reach, though ...