The latest release of PIE is used for IE7 and IE8 but we had some issues with it in IE6 where it would throw JavaScript errors. In addition, there is a problem with hidden elements (style="display: none") for example PNGs with transparent backgrounds or elements that have a linear background and rounded corners. If those become visible without reloading the page, the styles might not be applied correctly. You may find black images instead of the PNGs or missing element styles. This cannot be reproduced regularly, though. I found a beta version somewhere in the message boards that fixes the issues but does not work well with other versions of IE.
We never had any performance issues although we quite heavily use layout elements from CSS3.
BTW: The main website can be reached here http://css3pie.com