Previous to the nerf in U5, TWF was 100% offhand proc rate for everyone, and taking the three TWF feats gave you extra hook attacks, plus Tempest itself gave 10% and more attacks. There is a rather large disparity in the rate of attack with normal weapons compared to where it used to be. There are more ways to get doublestrike however, and many of them stack. Some of that disparity is made up due to better gear.
6 levels of ranger used to be the best deep splash in the game. 10% haste (back when offhand was 100% proc), rams, 2 of the 3 twf feats, many shot, bow str, rapid shot, and 2 favored enemy. You had to burn three feats to get that haste, but it was worth it.