This will help lower latency in the system, giving a better overall feel of the desktop.
Well, I don't fully understand what DirectFB is, or how it works, but if it does what I think it does -- gives X hardware acceleration -- then that would speed up everything that uses the GUI at all.

I especially like the idea of building everything on a supporting framebuffer, which uses software emulation to support all the neat stuff for cards that don't support it.
That's kind of good, just so people like me with bad hardware aren't left out, though trying to render stuff like that alphablending in software mode would probably bring my P-II 300MHz to it's knees.