there is still debris coming off the wheel when it hits the Halo. I suppose its better than to have the whole wheel hitting the driver, but at 250kph, that piece of rim will penetrate the visor easily.
I think along with the halo, they should maybe consider windows (don't flame me for this guys). If they want it safer, put fighter jet style glass in, not the top of the halo (of course), but all the halo currently does is stop big pieces. Massa will still get his ass handed to him by that spring in 2009 even with the halo. The bullet proof glass is key to this thing actually making the most sense. If you want to stop debris from the sides/front, the do it properly by stopping the shrapnel as well.
Just my opinion