I downloaded the Command & Conquer 3 Demo today for the X-Box 360.
I finished the Training Mode and stomped 2 guys on-line.
As an avid fan of the series on the PC platform I can honestly say that the 360 version isn't as bad as I originally feared. It's certainly not as efficient as the keyboard and mouse, but not at all impossible to get the hang of.
It appears that you can only play as GDI in the Demo and only one map in versus mode, but it's better than nothing.