i havent enchanted with fifty lvls yet but i enchanted with 40-something once and got smite five. i havent used it yet because i was already halfway through a sword when i got to the point that i wanted to enchant, so its just sitting in my "really cool stuff" chest with gold apples, saddles, and records.
I just love the Knockback enchantment! You can littarly send mobs flying!
And it's very usefull for fighting creepers.
And Looting is very cool too, enspecially if you need ender pearls (I'm having trouble finding those things!).