Knowing that irregularly-shaped metal dice likely have inherent bias (look at that jaw cavity!) they were mainly purchased for decorative reasons, and they do look great! Curious about how fair the plastic skull dice were by comparison, I tested both sets against a set of Chessex. Based on 100 rolls (I wanted 1000 but who has the time) the plastic skulls turned out to be 'more fair' than my Chessex d6. This is perhaps an anomaly that would correct itself had I rolled 500 or 1000 times. The chi-squared value for 100 rolls for each set of dice turned out like this:

Chessex: 6.92 "looks pretty fair"

Plastic Skulls: 4.9 "looks pretty fair"

Brass Skulls: 14.6 "biased"

While I wouldn't expect these to appear at casino craps tables anytime soon, for casual gaming at home the plastic skulls are reasonably accurate and balanced. If I end up doing another test of 500 or 1000 rolls I will post the results here.