1) Good weather throughout the year (hovering around 26-28 degrees)
2) Beautiful landscape with mountains, plains, lake tanganyika that provides leisure and possibility to develop sea activities (jet skying, etc)
3) Welcoming people and easy to get along with
4) This year, Burundi has been ranked first among African countries that have conducted important reforms to improve the business environment (source: 2012 World Bank's Doing business) - now It takes only one day to create a business
5) gateway to other east african countries (Kenya, Uganda, Tanzania and Rwanda)
The number of expatriates in the country is rising exponentially every year, this is a good indicator of the quality of life you can get there.
However I must say that the country is yet to resolve the problem of power shortages.
Bottom line, it is a good destination but it also depends of what you want to do over there... If you are an entrepreneur than it is the best as it is a virgin land for almost everything!