Having a niche blog is good. I made a few niche blog until on time i feel that I cannot catch up with my blogs.
I try to make a niche blog based on twitter. However it does not work well to me. The main reason is because the topic are too broad and too fast. I can follow the topic, and latest new in my niche but to write requires time. Sometimes there are few news to be covered. Having a niche blog which are not up to date is basically not very good practice.
Now I'm focusing on niche blog which are not too fast and very specific. The visitor maybe not very high since it is too focused, but I believe it will converts very well.