The SEO community seems to be split on this issues as well. It's hard to decide, given the fact that targeting a few very popular keywords could bring to your site thousands of visitors a day, but at the same time you would be facing some very stiff competition. The more popular a keyword phrase is, the more optimization and link building effort is needed to achieve top ranking. So there is a trade off between choosing less competitive phrases with lower traffic vs. more popular keywords with higher difficulty levels to attain top ranking.
