Hi,
My question is not really specific but i need opinions from the mosts experienced :) (again)
I start enjoing drupal there is one or two years now and i join a big drupal 6 project there is some few month.
There are thinking of upgrading it now, but it's a hudge beast, it will take a loooooong time, so we have to continue to maintain this one until the new fork is ready.
The best reason for upgrade, more than security, is because it's heavy. VERY heavy: 5 or 8s load pages on the production optimized server, up to 24s on my developpement local machin !