Exactly zero. I've never thought the reward for getting a Wither Star was worth the effort involved in going after one. As much as I like Minecraft, the bosses and the rewards for fighting them is one of my least favorite parts of the game.
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None; I've never had the inclination to make a Wither in Survival (I've made one in Creative and killed it in Survival just to see what it was like), mainly because of the time needed to collect enough skulls, the lack of an achievement for it in 1.6.4 (I do try to get all achievements before the "end-game"; "on a rail" is the only achievement I haven't gotten in every world as I only get it if I explore far enough away to build a secondary base), and the reward not being that useful for my playstyle (at best, it might let me move around my base a bit faster, but I spend nearly all my time away and 100x100 blocks isn't much when I cover several times that area in a single play session while caving, and in random directions).