It's good to see that the performance that we lost in Windows XP with the HD 3870 X2 is back, but apart from that the Catalyst 8.4 driver on a whole isn't that exciting. The HD 3870 manages to lose a bit of performance in a number of applications while the HD 3650 saw some gains under Vista in our OpenGL based benchmarks.
X2 Windows XP users might take the chance to upgrade the drivers since they missed the last lot, and HD 3650 users may as well take the upgrade since performance under OpenGL has gone up a little without any drops elsewhere.
HD 3870 users might choose to skip the driver with a slight performance decrease in a number of applications. The only reason that you probably want to update to the latest Catalyst driver is if one of the bug fixes are important to you.