As Piem, i would say it's a good buy.
The new possibilities cover a lot of things of the "core" of the application and are really worth the upgrade.
That is if Carrara was enough for your need.
If you need what can only be done in Maya or XSI well... Buy XSI or Maya as Carrara 6 won't be the right app !
Sorry, just joking.
All in all, go for it, the C6 upgrade was a lot more than just DAZ content oriented.
Cheers,
Laurent aka Tartiflette