I read an interesting article yesterday about these and POSSIBLE issues with drowning with inexperienced snorkelers and knock off masks. I have never used one and have zero experience or a good or bad opinion.

Issues raised and make your own opinion.
1. CO2 buildup in mask
2. Panic when mask floods and not able to get the mask off quickly.
3. Are the deaths result of just out of shape people over exerting themselves?

Google full mask drowning to find info.