That blasted push-pin mechanism is horrible. Seriously, flimsy little plastic pins being used to hold a critical component onto your CPU? I can't for the life of me install those stock heatsinks properly. I'll get one leg down and locked, and as soon as I try to secure another leg, the first pops back out. Wow, that's confidence inspiring! I have a harder time using those push-pins than pretty much any other type of retention mechanism. Even those plastic Dell brackets are easy to install a heatsink in. If I ever get an aftermarket heatsink that has the push-pin mechanism, I'll be sure to buy the Thermalright bolt-thru kit to go along with it.
The only time I use that stock heatsink is when I have a bare board sitting on top of my desk, in which case, I'm usually just too lazy to dig up the various bolts and screws for my "real" heatsinks. In any configuration where I have a motherboard sitting vertically though, I wouldn't trust that heatsink one bit. Can you imagine if the Scythe Orochi used push-pins? That's the stuff nightmares are made of :Q...
/rant