6.2-6.3 went smoothly for me and elimated one the hacks I had installed. I haven't upgraded to 6.4 yet. I probally won't upgrade until 6.5(which will elimate the need for a couple more hacks), money thing. But, there are just 2 basics when upgrading. Backup your database, and backup your files. I only backed up my database the first time I did it and blew out my board rules, custom headers, etc. and someone(I'm not naming names, but this person now has blue hair) deleted my more recent local copy and I was left with only an old archive that didn't have alot of my custom stuff.
Also, do a practice upgrade first, if you can. Duplicate your database and make another install on site adjusting the config file. Then run the upgrade script on your test DB and then upload you 6.4 files. That should let you know if you are going to have any problems upgrading without putting your production site in any peril.
My 2 cents.
