IMMHO, it is never a good idea to upgrade to a new OS without first having made a verified backup (clone) of your existing system. I have used the upgrade option over and over with no problems but you never know. There are so many things that
could go wrong and you never know for sure about third party apps, drivers, etc. You don't even really know if you will
like the new OS enough to abandon the previous version until you get it up and running. So I always clone my system and upgrade the clone first. Once I decide it's a "keeper" I simply clone the upgraded system back to my primary drive. I also make a copy of the original clone on another drive just to be extra safe.
I think it is important to be realistic about this. So many people who are unfortunate enough to have issues immediately jump all over Apple and accuse them of being careless and unconcerned when nothing could be farther from the truth. But, more often than not, these same people ignore the fact that these computers and operating systems are incredibly complex and consistently fail to perform even the most basic backup of their most valuable data. Invariably, these are the ones who scream the loudest about Apple having ruined their lives when something goes wrong.
There was a post by some guy who blindly upgraded a "mission critical" machine when he was on a tight deadline and the guy acted insulted when someone suggested that this might not have been the wisest approach. Of course, he had no backups and, of course, he blamed apple for that too....
