I've got the same trunk as twahl, I've had it for a couple months. I've also got the rear rack from performance. It's rated at around 40lbs or so, but it can hold a helluva lot more than that. I once lowered my seat and sat on the rack and rode around lowrider style. It held my entire body weight just fine, but I didn't ride like that for long and didn't hit any bumps.
The only thing about the performance trunk is that now the walls are starting to lose their stiffness and it sways back and forth quite a bit. This just started happening recently, now I just run a bungee over the top of it and it holds still. That trunk can hold a lot of stuff, i'm really happy with it. Not bad for $20!