One positive when using a leader: You can use a lighter leader than your mainline, so that you're less likely to lose your float when you hang up.

One negative when using a leader: Like Jigman said, your knot is the weak link and now you have two of them, instead of just one.

I guess it just depends on how much you want to spend on floats, and how much faith you have in your knots .

Fish on.......
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She was standin' alone over by the juke box, like she'd something to sell.
I said "baby, what's the goin' price?" She told me to go to hell.

Bon Scott - Shot Down in Flames