I'm the publisher, and I'd be happy to answer questions. Let me see if I can give you a basic answer, though. For a well-prepared party with a good healer, it doesn't raise lethality too much. It's hard to get a crit that kills instantly. Most kill in several rounds. The main thing it does is allow a crit to have a big tactical effect on combat. If your fighting and your defender gets hit with a big critical effect, the cleric has to redirect his attention to getting him back up (especially if there's a clock over the party's head from the effect, such as he'll die in five rounds if they don't heal all the damage). Most of the horrible stuff (like dying and stun) can be fixed by healing all the damage caused by the crit and the effect. The permanent effects (like maiming and paralysis) can be fixed with one of two rituals inside.
The big effect it's had on my playtests is that it heightens the excitement of a crit. When that 20 comes up, all talk at the table stops. Everyone leans forward while I pick up the book. If I'm in a store, employees stop to hear the result (since they know what a crit means when I'm DMing). If it's against a bad guy, they're all ready to cheer. If it's against a PC, they're dreading the big roll that might shatter their ally's arm.
Most of them say that it turns criticals from boring into exciting.
Ask questions. I'll answer as I can.
