They're all pretty easy to install. I suppose the Prism is arguably the easiest since you just clip it on and then flip the tightening mechanism, but the Stealth and Spire are not hard either.
With the ones that screw in (Spire/Stealth), just be sure to hold it down with your off hand while you start tightening. Start with one screw then go across diagonally in an X pattern. Start out with just five turns in each screw to get them to engage. Don't tighten all the way down until you've made sure all screws have engaged, then do another five turns in each, and then tighten all the way.
The people who had trouble probably were trying to tighten each screw all the way down before moving onto the next or something. That would make it basically impossible to install.
That being said, the Prism is the only one of those heatsinks that's worth a damn, so if you're buying for the stock heatsink be sure to get something with the Prism.