I haven't picked up the game yet, but I've noticed that the core set seems like quite the deal compared the expansions. Especially when you consider the dice, templates and such.
So my question is this, when do you hit diminishing returns on the core sets? I know there are some pieces in the expansions that don't come in the core sets, but I'm still thinking maybe 3 core sets, plus whatever expansions end up fitting my play style.