For a single-class combat character (high BAB progression) you'll hit the required BAB at level 8. For most characters that do their damage through straight up bashing things with weapons it's an excellent feat, since as Hickory already explained it can greatly increase the amount of crits your characters are dealing. However, keep in mind that certain types of damage, such as sneak attack damage and elemental damage on weapons does not get multiplied on critical hits, so any characters relying heavily on these as their source of damage won't benefit much from the higher crit rate.

Something to also be aware of is that you can get the same effect by enchanting a weapon with the Keen property (or casting Keen Edge, although this is only temporary). Keen and Improved Critical don't stack, so if you're planning on using Keen weapons then there's no point in taking Improved Critical.