I'm personally more interested in these:
http://www.begadishop.com/sheets/bcb...ew-english.pdf
It seems they should be much more consistent than plastic, since they will have next to no internal differences (bubbles, uneven density distribution) compared to plastic, as well as being easier to visually inspect and being more resistant to deformation/scratching from passing through metal feed lips and the like.
No shatter danger unless you're shooting point blank against a solid object with 3 joules (over 570 fps with a .20g). If someone is shooting you that close with that much power... I don't think I'd be worrying about the glass BB as much as I'd be worrying about getting that player off the frakkin' field.
Anyone want to give em a try? The review raves about their precision; I wonder if they'd give .29g SGMs a run for their money.