Recently I upgraded our several years old OTRS install to version 5. The upgrade procedure is exactly like all the others have been. It was straightforward and I actually found that the documentation was better this time around than it has been in the past. After backing up, upgrading and testing, I like, mostly, what I see. There are a couple of things that are a bit annoying though.
When closing a ticket, the previous version would automatically put in subject in the closure note. Likewise, in the Owner Update note it doesn't do it either. So each time, I have to update it. Not a problem if you're only getting a few notes every now and then, but we are changing and closing tickets in some large numbers at times.
Also, I have about 50 statistics generated monthly via a script I run. The syntax for doing this.
From: /opt/otrs/bin/otrs.GenerateStats.pl -n number -blah blah etc
To: /opt/otrs/bin/otrs.Console.pl Maint::Stats::Generate --number number etc
I like the speed of the updated system and the cosmetic changes are nice too. The menus have changed slightly and they are more dynamic with drop downs. The layout is a bit nicer too but generally the speed of the system is better.
Upgrade to version 5 if you have the chance - seems to be a winner!