Two Weapon Fighting in Pathfinder
In this, it's argued that two-weapon fighting is a terrible choice for a ranger because fighting with paired short swords yields the same max damage as an identical ranger without the two weapon fighting feats holding a greatsword. This has a couple flaws with it. First, pair short swords are not nearly as good as longsword/short sword. Second, it's not max damage, but expected damage that matters.
I did some math on a few builds. I assumed the two-weapon fighting ranger takes two-weapon fighting at first level and double slice at second. At 6th level, he takes greater two-weapon fighting. He's using a longsword and a short sword. The greatsword wielder has no feats at all, as far as we're concerned. Both characters have 18 strength. Here's the expected damage for each against ACs from 10 to 30.
As you can see, first level still gives the advantage to the greatsword wielder. However, by level 2 (and the third feat) the two-weapon fighter has passed the greatsword at low ACs, if only by a little. Then the advantage goes back to greatsword for very high AC foes. The greatsword holds that slight advantage until level 6, at which point improved TWF gives the two-weapon fighter a significant advantage below 24 AC and a slight overall advantage. I only went up to level 8, since much past that and a no-feats greatsword wielder is at a pretty huge disadvantage. Level 10 opens up greater TWF and two-weapon rend, which cinch things.