When Adobe released CF8, they touted the increased performance. I've been using CF since 4.0, and I think this has been a claim for every release. I assumed it was just a marketing ploy, as I haven't noticed a huge difference in past upgrades. However, I am AMAZED at how much faster CF8 is than CF7.
We have a very programming-intensive portal that is heavily used by pretty much everyone at the college I work for. Its the access gateway to almost every Web system we have. Prior to CF8, the average page processing time for the portal was about 700ms. After CF8, it is down around 200ms. While a half-second might not seem like much, the impact is enormous. As a result, the rest of our site runs much faster. If you multiply that half-second by the thousands of logins we get in a day, you can see how much processing time is saved.
Our whole site is noticeably faster, even at peak traffic times. This boost alone makes the upgrade worth it.