I agree with berner. I've actually had a couple of powerlinks let loose. Not break, but just fall off. And it wasn't because I installed them wrong - this happened after at least 100-200 miles after I put them back on. Both times, the links were fairly old-ish (I don't know how old), reused many times. Now, I make sure I never use a link on more than one chain so its life isn't ever extended beyond a typical life of a chain for me.

I don't know if this applies to the Shimano 8 speed chain you have, but I thought I'd mention it. I had a shimano 8 speed chain a couple of summers ago that wouldn't accept a Sram 8 speed link. At the time, someone told me something to the effect that there's two versions of Shimano 8 speed chains and one is just a mm or so too wide for a Sram link. Hopefully someone with expertise will chime in and clarify this if anyone's interested. I don't have any 8 speed bikes any more so the info has left my brain except for a memory of the hour or so I spent trying to get that damn link on that chain!