Can Aikido be used for self-defense?

Yes, Aikido can be a very effective form of self-defense. However, it can take considerable time and effort before Aikido, or any other martial art can be used effectively in a self-defense situation.

Does Aikido take longer time to master and apply than other martial arts?

The simple answer is “yes”. A year in Karate/Tae Kwon Do/Kempo and you can probably fight much better than before. It takes well over a year before you start feeling comfortable enough with Aikido techniques to imagine using them in “real life situation”.
The complex answer is “no” in the sense that one should never feel like they have “mastered” an art. If they do then they’ve stopped growing. Martial arts is a way of life.

Is Aikido better than karate/judo/any other martial art?

The answer to this question is very subjective. There are many different but equally valid reasons for studying a martial art, such as for self defense, spiritual growth or enlightenment, general physical health, self-confidence etc. Different martial arts, and different styles within a particular martial art, emphasize different aspects. It’s a personal choice.

Does Aikido have competitions?

O-Sensei felt that competition was incompatible with Aikido, therefore the majority of Aikido schools do not have any competition.
Most Aikido training, even in schools with competitions, is of a co-operative rather than antagonistic nature, with both thrower (nage) and throwee (uke) working as partners and trying to optimize the experience of the other.
This working partnership is also necessary to minimize the chance of injury from practicing potentially dangerous aikido techniques, and to develop both partners’ capacity to “take ukemi”, to be relaxed and able to take care of oneself when responding to “falling” or being thrown in a martial situation.