I have just come across CSS3 pie.
I was wondering how reliable it is to use? Will it work on all operating systems and are there any scenarios where it may not work?
I'd imagine it's much more about browser than OS. This is why "virtualized" web front ends like Flex/Silverlight/JavaFX are popular now because all these concerns mostly go away.
It's difficult to say how reliable it is since it's ultimately reliant on the chaotic ever-changing browser environment. Something could break on the next update of IE/Firefox/Opera/Chrome/Safari, or it could work fine. (Actually it says it's only for IE 6-8, so if you're not okay with that, you may run into trouble?)
It'll probably (?) be hard finding someone with actual experience with this util. Don't forget to check the list of known issues: http://css3pie.com/documentation/known-issues/