I'm a big fan of simple when it comes to fishing, and when it comes to leaders, it doesn't get any more simple than the Ian Colin James formula. If you haven't seen it,
here it is.
My personal leader is slightly more complicated. Basically, you take a three foot piece of 15 pound maxima chameleon and attach to it a 1 foot section of 15 lb yellow amnesia. Afterwards comes another 1 foot section of maxima followed by a 1 foot piece of red amnesia. The red amnesia is 10 or 12 lb test. The yellow and red amnesia serve as strike indicators. If I'm fishing in a stream that contains sewage after a heavy rain, I'll attach around 5 more feet of 6 lb Berkley Vanish before the fly. If there happen to be trout in the stream I'm fishing, I'll put on 3 feet of 6 lb Vanish followed by 2 or 3 feet of 4 lb Seaguar Abrazx. The Abrazx is .007" diameter making it 4X.
The Vanish is cheap at about $9 a spool, but the Seaguar runs about $20. Both are flourocarbon and are far cheaper than buying "tippet" material. Personally, I like the 4 lb Seaguar for everything including carp; however, the carp don't seem to mind the 6 lb Vanish, and at $9 a spool, I would rather use it.
The moral of the story is to keep it simple and save your money.