I can't help but shake my head at anyone who would spend half the cost of the Peak itself on an insert made in China.
I've had the opportunity to try out a Fadespace and an EQ insert, and neither seemed noticeably better to me than the stock ceramic cup.
What's made the biggest improvement to me so far has been using a better bubble cap (I got one from spacewalk).
I also haven't had an opportunity to try anything other than stock glass so far. $200+ to mod this mechanically basic and soon-to-be-obsolete device just isn't reasonable. Also, one of my favorite things about the Peak is the carrying case, which won't fit anything other than stock glass