So I do not think it is maximized as it is extra damage from a magical weapon granted on a critical hit. I would call [W] a die roll.
So would the bonus apply to implement attacks using the weapon? Even at heroic levels (crit on a 20) the numbers could get big fast with repeated area attacks.
Well, it's a melee weapon enchantment, not an implement. Granted, a Swordmage could use it, but it's not like a Staff Wizard that's throwing out constant area powers would have access to that enchantment. Plus, it only lasts to the end of the encounter. Even if you attack 5 guys a round for 8 rounds (40 attacks), you're only doing 2 crits (since it's essentially a 1-in-20 chance to crit), which means that you only use the property one time (it's +1[W] on your next crit).
. . .and said Sorcerer could be a crit-fishing Daggermaster, but since it increases the damage by 1[w], I'm not sure how that would interact.
It looks good for other crit-fishers though, like an Avenger or something. Increasing your crits by a d12 cumulatively sounds good.
Jay