Fartlek is intended to be a type of interval training without fixed distances, times or paces.
Is this more effective than regular distance and interval training in separate workouts?
To me, fartlek is not just fun. As our body is a very adaptive system, if one sticks to a training routine long enough, his body gets used to it, and thus will not make much improvement. So variation is the key (at least to me and itself is fun) so that fartlek can trick the body to keep adapting to different training intensity and condition, thus it can be effective than, but not replace, regular training.
Fartlek is not more effective than any proper interval training - just more fun! After all, you decide when to speed up during the run and for how much and how long... And in my experience that is not nearly as hard as when you run against the clock.
(I believe "fartlek" is derived from Danish "fartleg", which means "speed game"...)
EDIT I stand corrected. As noted by @Henrik "fartlek" is likely directly from Swedish...
We only used Fartlek as a fitness measurement, and as a bit of fun.
It does train you, but not as well as a more rounded fitness regime. It is, however, a very good indicator of short term fitness and speed, so is useful as part of your training plan at intervals of a month or two to help you gauge progress.