Like, HeapThaumaturgist I have used a couple of WoL as well as a few Items of Legacy. The rules themselves a really balanced, the only thing to really worry about is adding the "player" element into the mix. If using on of the pregenerated WoL there is no problems, but if your making your own there should be 3 things to remember.
1) The item becoming a WoL: This shouldnt be easy or caused by "every day" adventuring events.
2) The powers of the WoL: These should accurately follow a theme; whether its the build the character, the theme of the campaign, or focused around the event that gave the WoL birth.
3) Balancing the Powers: Everyone loves having a kick ass weapon and/or item. The DM should be part of the entire process of creating and adding to the WoL. This will help keep the WoL balanced and prevent the rest of the party from getting envious of the player who does have the WoL.