Let's not all forget, upgrading a live site with no backups is a pure rookie mistake. Any competent admin will attempt the upgrade on his sandbox first. If you don't use a sandbox, then at least have a backup plan.
Come on "MDawg" this is administrative 101. If your upgrade didn't go as planned, you only have yourself to blame.

I as well was running 3.8.4 and decided to go with the upgrade I did do a backup yet all my old threads and user info was lost! I tried to restore but nada! Other then some small issues my members that have RE-JOINED are loving the new upgrades and are patiently awaiting for those old mods we had to be upgraded to work on our site

I had 3.6 before, I did a fresh install of 4.0, used Impex to import all users and posts, and I installed new mods and tweaked the new styles, everything is great now.

But, 4.0 does seem to run a little slower. =(

Rule #1

Copy your database, copy your forum folder.
rename the new forum folder to forum2
in the config, point the database of the forum2 to the copied database.

You now have a clone of your site. Do all upgrades on it. If all goes well, just rename your forum2 to forum or whatever the old one was and youre done.

If it all goes kaput, you have nothing to worry since it wasnt your original.

Yes, I find it a lot slower...I mean a lot!!!! strange!!!?? anyone know why this is?

The upgrade process was pretty smooth for me. I removed all products, I cloned my database, and worked off the original database with the cloned copy as a backup. Then I moved the 3.8.4 forum directory to another folder within my file system, and rooted the new CMS over top. After that I upgraded via the install scripts. Went well for me.