I am currently testing in the hope of replacing a Lotus Domino server for 350 users, but i havent used 5.6 much, and i dont think 6.0 works perfectly out of the box either, not unless your already fairly experienced with SME, which i am not.
I am still tossing up whether to continue with SME or to either use Red Hat 9 or Fedora, RH7.3 is starting to feel a little for a new implementation...
Do test it thoroughly, and find yourself a script that will allow you set up a test machine and PUMP thousands of mails per hour, simulate webmail usage on a large scale etc before you commit...test test test