Arguably the most recognized weapon in the world, and by far the most widely used, the Aktomat-Kalishnikov Model 1947 was developed by Russia as a multi-use high powered weapon. Highly durable, the AK47 has withstood the test of time. Whereas the M16 is constantly upgraded (and redesigned for the M4), the AK47 is still produced as it was 63 years ago. New models are occasionally released with lighter components (AK102 being the most recent), but the design remains the same.
The AK47 is a great all-around weapon. Like the M16, it is best fired in bursts, however it is by design better at firing fully automatic than the M16. It makes up for lower accuracy by being very powerful, able to take out enemies in 1-2 bursts depending on range. At medium or close range, it can be fired continuously to dramatic effect, though again this would most likely just waste ammo, considering the gun's power.
Really, there is no reason NOT to use the AK47. It is the most well-balanced of the three assault weapons. Unless someone is better suited to the M14/M16, or using them for specific reasons (see their respective pages for strategies), players should and ill generally choose the AK47. Luckily, because the weapon is available to all immediately, there are no issues with balance, as the Marksman ability has little effect on the AK's recoil.