As with any upgrade of Logic or the Mac OS , many 3rd party plugins stop working.
I recommend installing this on another hard drive with a copy of your OS.
I use Carbon Copy Cloner to make an exact copy of my system on a different hard drive. Then I install the update.
I've opened a few songs from Logic 8 and like I said some 3rd party plugins don't work.
There are some really cool things in Logic 9 but I think I'll till Snow Leopard comes out in October to start using it as my main system. Hopefully by then all the 3rd party software will work well.
At that time I'll do a clean install of the OS and Logic 9.
Keeping Logic 8 and Leopard on a old hard drive for backup.