My mask leaked for a while and it took some work to get the lenses in properly.  I wish I went up a diopter but I can't find them on the Swimoutlet.com website anymore

They work very well and keep the water out of my sinus.

+  Black silicone skirt blocks skirt light from reflecting off the lens and reducing visibility.
+  The lenses are tempered glass to resist scratching and allow thorough cleaning.
+ + +  IST Corona is one of the very few masks available with positive diopter lenses, and maybe the only one available with glass positive diopter. Get a spare mask with standard lenses and save a bundle when your optical mask gets a broken buckle or the cockroaches get an opportunity and eat the edges of the water seal.
-  You must thoroughly scrub the new lens with toothpaste or a paste of baking soda and water to remove the coating on the lens or it will be plagued with fogging. (Note: If you do this to a PLASTIC lens, like SeaVision's,  it will be ruined!)

The lenses were fairly simple to change, although there were no directions on mask disassembly.  Once in place there are no diopter markings on the lenses, so if I have to change lenses again it will be more difficult getting the correct lense in the correct place.
Overall it's a good mask for the money.