A major upgrade to Mac OS X comes out tomorrow. If you are considering upgrading, here's what you should know:
The first BIG thing about Lion you should know is that Rosetta is no more. This means that any and all PowerPC apps you have will no longer work. This is not a big deal to some, but if you run Quicken or Canvas, this could be a deal killer. The very first thing you should do before buying Lion is choose "About this Macintosh" from the Apple menu and click "more info" and then click on "Applications" under Software. Sort by kind and look for anything labelled PowerPC. Unless you have only had your mac a very short time, there will likely be some things in there that will surprise you.
For me on my desktop it is mostly a realization that Office 2004 will no longer work, and I'll have to make peace with Office 2011 (never did like 2008). The other loss is Canvas X. Canvas used to be one of my primary tools, until it was purchased by ACD who dropped the mac product. Now it's a sorry sad sack of an app long in disrepair. Lion will finally deliver the coup de grace to that once proud product. ACD's solution is "use the windows version" which is a bit like spitting on the mac version's grave to me... My laptop has a larger issue, which is quicken 2006. I use features that are not available in the much maligned Quicken Essentials for Mac. The switch to Lion is going to force me to finally move to a web based online banking and abandon quicken forever. Perhaps its for the best, Intuit's heart hasn't been in quicken development in years. These things are manageable but important that they be considered BEFORE making the switch. Once Lion is running, these apps will not even launch anymore. If I want to convert old Canvas files or export things from Quicken, I need to do it before I make the plunge. (An aside: the licensing for Lion allows mac os x to run as a client os inside virtual machines on Lion. Hopefully this means that we can setup VM's of snow leopard to run Rosetta apps and possibly even Tiger to run classic apps, but since currently that isn't possible, we'll wait and see how that pans out).
But enough about the loss of Rosetta. Once I am ready to make the move I plan to do a total nuke and repour, which is my standard operating procedure for significant OS upgrades. This means: (1) Image off the entire drive to an external drive. (2) Test said image and make sure it mounts and reads properly. (3) Make a copy of that image to a second location because I'm paranoid. (4) Format my internal hard drive and install from scratch. In the case of Lion it means I have to install a copy of snow leopard and then install lion from the app store. By the time I actually do the install, I hope there will be a way to skip the install snow leopard step, but that hasn't been announced or promised. The official solution for macs that can run snow leopard might always involve an install of snow leopard first. (5) Download Snow Leopard from the App Store and install. This is not trivial, as it's going to be a LARGE download. Plan a lot of time for this step.
If I have the time, I like to reinstall EVERYTHING on my machine after a major upgrade. This is by most accounts, insane. But I like to insure that, at least for a short period of time, I have all of the latest versions of everything and there is no data corruption anywhere to worry about. I haven't done this for a while, so Lion is probably a good opportunity. It's a lot of work, but the reward is a machine that runs trouble free for a long, long time. Once I'm done with this I don't expect to do anything to my machines until 10.8 is released, whenever that is.