Predictably, you’ll read lots of articles over the next few days about the proper Mountain Lion installation strategy. On the official day of release, you’ll read early reviews here and elsewhere as to whether OS 10.8 fulfills all or most of Apple’s promises.
You may also hear chatter from early adopters who discovered the usual round of point-zero bugs. Some might be benign, others might be serious. Some people will report problems with anything, and there are still reports that Lion remains fatally flawed even as its successor is ready to launch.
While I will always urge caution about any OS upgrade, even on a Mac where the process is supposed to “just work,” my own upgrade strategy tends to be more casual. Since I have a clone backup of my iMac’s startup drive — a complete duplicate — I would not lose anything but a few hours of time should the upgrade not go smoothly.