the truth
Unfortunately, most people overestimate the value of improved critical. It is most useful against opponents with low armor classes. This stems from the fact that you must actually hit them to turn your threat into a critical. If you go up against someone with an armor class of 1000 (yes, i know, its ridiculous, just using it for the point), you will never ever critical against them. The best you get is auto-hit. Even if you have a crit range of 1-20, it won't matter. And since you can't take improved critical until your base attack bonus is what.. +8 i think? that means that by the time you take it, you're dealing with creature whose armor classes are well into the twenties.
If you do the math on it, it turns out that weapon focus has a greater effect on average damage per round than improved critical does.
Now, comes my disclaimer. There are several factors which make Improved Critical better for certain characters. If your attack bonuses are very high, you are more likely to turn that threat into a critical... thereby making imp. crit. more valuable. Also, the feat has a larger impact on weapons with an already large crit. range. So, if you can get a weapon with a high crit range (say 18-20) to begin with, add into that high attack bonuses, then improved critical becomes worthwhile. Otherwise, if you don't have good attack bonuses, and are using a threat range 20 weapon, the benefits are really very small.