I think it's like any other genre of historical writing; if done well, it's good, if poorly then not so much. Sorry to sound reductionist, but if you consider, say, this and this to be both microhistories, well, one of them is a pretty groundbreaking work that can advance its genre, and the other is a fun beach read about a fish.

Those two questions set up an unnecessary binary. Some forms of microhistory have been overdone (like the trend among early Americanists in the 80s and 90s to do micro-level studies of virtually every colonial New England town). Others are only beginning to be explored (like James Sweet's Domingos Álvares, African Healing, and the Intellectual History of the Atlantic World, a study of Inquisition documents relating to an African healer). Generally though I see the field moving away from microhistories because digital methodologies lend themselves toward bigger-picture analysis.

What is microhistory for the uninformed?

The way I read about it, microhistory itself is not about expanding into new fields but about reinforcing the relevance of other methods such as regional history und Alltagsgeschichte. These views of history have the imminent weakness of a focus that is too small to really "tell us something about what it was like in general". Microhistory for me is the method of finding ways to make them count.

Microhistory is sort of the combination between Geertz/Levi inspired cultural history and heavy, quantitative history, and in that way it is a bit regressive. However, when properly employed, microhistoric analysis can be a useful tool in adding to new fields as well as countering widely held historic generalizations. It really depends on the skill of the researcher to wield the tool, than it does on the genre as a whole to determine its fate.