I know others have had success with clean installs but I didn't believe it. I took my mid-2009 MBP into Genius Bar last week with dead superdrive and unbelievable slowness and lagging overall. Lots of beach balls of death, program crashes, system hangs, slow browsing, and about 5 minutes boot up time at least. At first they thought it was the logic board as their tests showed possible sensor issues. They investigated further and found it was a software issue. I knew in advance there was at least a dozen disk permission errors that DU wouldn't fix. I also knew my recovery drive was missing. The solution was to do a totally clean install of Lion. They recommended this and further recommended that I don't restore from Time Machine automatically and do a manual migration of files and to not reinstall any applications off time machine. Wiped the drive, made a bootable Lion DVD and put it on fresh. No Leopard to Snow Leopard to Lion install, just direct to Lion. Restored my user name,settings, and files. Used the Roaring Apps website to check application compatibility and manually reinstalled applications one by one. I did not reinstall any Adobe applications (like Photoshop), old programs, or questionably obtained applications. While I don't find it fully acceptable that the Genius Bar's answer to my problems was that "Do a clean install and manually reinstall programs you use because one or more programs is causing a problem with Lion making it real slow," I have to admit they were likely 100% correct. Hopefully Mountain Lion will go a lot smoother and be compatible with more programs. It's like a whole new machine. It's a lot faster, smoother, and I've had maybe one beach ball of death in 4 days. I plan on getting apps back on over time but for now, the clean install and installing only the apps you need has made a world of difference.