Have you perused the following from Apple?
I think it behoves us to stop blaming Apple for problems we cause by installing apps/hacks that use hooks that Apple has always said may change or that it may not support in the future. Many of these things are extremely useful but we use them at our own risk. Upgrading without first disabling them is asking for trouble, IMHO.
The only sure-fire method of painless upgrading is to return the System to as stable and stock condition and creating as much free space as possible. Those and other successful steps can be found at many forums. Recovering from not so stock, limited disk space
is possible, but not without more work than it would have taken by doing it first.
Also, remember that 'help' sites will always have more reports of 'problems' than successes. People who have no problems seldom ever bother to report such, it ain't interesting news!
It's the reports of problems that provide the fodder for the Help sites. Thus 'problems' appear to be more wide-spread than their actual numbers. Secondly, most 'reports' are anecdotal and slim on facts and specific actions taken by the reporter. It is nearly impossible to supply a single fix for the almost infinite possible causes of the users problem, especially, when those causes may be unknown even to the user.
In particular,
hingyfan would be best advised to return to the last viable, stable backup and start over with all the knowledge now available. It may mean buying a new external drive to off-load extra files and even a complete re-install of a previous OS. At that point, the need to upgrade may actually disappear!