As people have said, it's only going to work if:
a) the niche is something the lawyer can actually be an expert at (you can position yourself as focusing on Polish/US intellectual property law but if you're not fluent in Polish and you don't have a science background who would hire you?)
b) there are people interested in hiring lawyers for the type of work you want to do. If there are other people who can do similar work (accountants, social workers) or if people can do it themselves, it won't work.
c) the people who want to hire you can afford to pay a lawyer (for example, you could have a niche practice representing homeless people whose belongings were seized from the sidewalk by the city, but your clients won't be able to pay you anything. You can do this work for a nonprofit but it won't be lucrative)
d) you are good at promoting yourself and building a brand.
here's a good article:
http://www.abajournal.com/magazine/article/building_niche_law_practice