Knotless leaders allow you the greatest ease of use. The tippet should be replaced after multiple fly changes.  Do not use up the tippet section and then start to cut into the midsection that will ruin the leader. Use a Surgeon Knot to replace the tippet.

7 1/2'

 Most of these leaders come with a loop on the butt end for a loop to loop connection to your fly line.  If you don't have a loop use a nail knot to connect the leader. Short length turns over easily.  Fly stays close to line when using sinking lines. Accurate

9'

 Good general length.  Allows sinking fly to sink deeper, quicker.

12

 Good for longer dead drifts.  Allows sinking fly to sink deeper, quicker.

Pro

 Easy to use.  

Con

 Heavy butt material tends to coil. Only available in standard lengths and sizes.