In my experience, in order to develop a property in Detroit, being in the city seems important. Things are changing there, for sure, but outside a couple places (midtown, corktown), it is difficult to hold a property without it being "swallowed up" by the neighborhood. Scrappers still break into any house with any amount of copper, appliances or anything of value. Tenants, other than college students, seem to be hard to manage. Before buying a house, I rented there for years and saw many cases where rent was not paid for months and the tenants left with the furnace and hot water heater on eviction. Or left in the middle of the night. It is important to have an advocate in the city or to be there oneself, and crucial to form relationships with nearby property owners in order to keep something from being trashed. Many houses are in various states of disrepair, and squatters are occupying many of them.
This is all speaking from my own experience, of course, and an investor may find opportunities that can be taken remotely (such as parcels of land or buying into development). Houses can be bought for cheap, but holding is the tricky part.
If anyone really is interested in investing here, there are certain areas that may do better than others due to what the city is doing or what individuals have done in their neighborhoods. A light rail system being currently installed, which may increase property values along Woodward Ave, and there are various alcoves of the city where artists or entrepreneurs have begun buying.
If anyone would like advice on navigating the yearly tax auction or has any more questions about Detroit in general, feel free to ask!