Milky Spore as Japanese Beetle control
Sadly, Milky Spore has not proved as effective against Japanese Beetle infestations as it was first hoped it might. For starters, Milky Spore only controls the larvae damaging lawns, and it is not effective against adults feeding on plant foliage. It would be great if treating the larvae translated to fewer adults but, unfortunately, Japanese Beetles are strong fliers, and even if one homeowner treats their lawn, the Japanese Beetle adults are capable of swooping in from elsewhere.
It's also a bit pricey, so before you pop for it, make sure that the damage in your lawn is, in fact, caused by larvae and not just a result of water stress or fungus. Pull on the grass where you have brown spots. Does the whole grass plant come up and show the roots eaten off and black? Or instead does just the grass blade come off and show discoloration like spots or a "pinch?" If it's the latter, the brown spots are more likely caused by a fungus not beetle larvae.
If you opt for the Milky Spore treatment, it should be applied in the spring, and you need to allow several years for it to build up in the soil. It would be fine to use it in conjunction with an anti-fungal treatment.
Here's a good fact sheet from the CSU Extension on Japanese Beetles which includes information about lawn treatments and Milky Spores: http://extension.colostate.edu/topic-areas/insects/japanese-beetle-5-601/