For example, MSA is used in formal speeches or interviews. However, just as soon as the speaker diverts away from his well-prepared speech in order to add a comment or respond to a question, the rate of colloquial usage in this speech increases dramatically. How much MSA versus colloquial is used depends on the speaker, the topic, the situation - amongst other factors. Whereas MSA is the same throughout the Arab world, the dialects vary according to the geographical location. The further away the countries, the greater the variation between the dialects.
Arab speakers form various parts of the world do communicate in their respective dialects.
With regards the first factor, Algerians may have more trouble communicating with Saudis than they would with Tunisians, as Algeria is much closer to Tunisia than it is to Saudi Arabia. With regards the second factor, the average Arab throughout the Arab world have no problem whatsoever understanding the Egyptian dialect as they are all exposed to it via TV programs, films and documentaries. Before venturing on a learning journey, you should choose the learning exercise that would match your objectives best. If your goal is to do research, be able to read Arabic books or media, or use Arabic in formal situations, then MSA is what you need to study. If you are going to use Arabic mainly in the spoken form in order to communicate with people on the streets, then colloquial Arabic may suffice.