I am stuck supporting IE8 sometimes, but there's generally no need for hacks - conditional comments are not hacks and I don't really see polyfills as hacks. I prefer to avoid them if possible, but it's not a hack in the sense that DrDoc is talking about (at least to my mind).
To me, the essence of a CSS hack is that it depends on a defect in the way the browser parses a CSS file (or perhaps a minority interpretation of the standard in some cases). The problem with that is that it's a secondary effect and not built for the purpose for which you're using it. It's a hack, a kludge, a workaround.
Conditional comments, on the other hand, if you must support IE, are pretty much future-proof and are totally standards compliant. Generally that's true with polyfills as well - they're using client-side scripting to perform a function that you wish was built into the browser, but they're not exploiting a defect in the browser.
BTW - I wish MS had not extended End of Life for XP. The main reason that people are still on IE 8 is because they're stuck on XP and IE is the max version on XP. When XP finally dies, IE8 will be dead. Since almost no large corps or institutions standardized on Vista, that means that when IE8 goes, so will IE 9. At that point, every browser in use will be very good with respect to CSS2 and a large portion of CSS3. Yay!