I have used tumblr for about a year now. It is extremely easy to upload and manage virtually any type of content. However, I don't think it is the best place to promote your beats. It seems that the overwhelming majority of tumblr users are female and 12-16 years old. The frequent users are not exactly going to be looking for new beats. On the other hand if you can figure out how to effectively promote your beats on tumblr, you will have very little competition.