A bad property manager can ruin a great investment and a great property manager can turn a very mediocre one into a cash cow.
Amongst other things, your management company should be responsible for:
Liaising with your tenant
Ensuring they pay on time
Organizing any repairs necessary
Communicating with you on a regular basis
Paying you the balance of your rent
Providing monthly statements
Placing new tenants.
A professional who manages your portfolio and your tenants well over a number of years is worth his or her weight in gold. The majority of property managers charge additional fees for placing a new tenant or renewing the lease of an existing tenant, so you should make yourself aware of these and input them into your calculations.
Your property manager is your eyes and ears on the ground so it is very important that you develop a good working relationship with them. They work for you so do not be afraid to ask questions and seek advice, but be polite and respectful.
Make sure your management company has a track record of keeping both tenants and landlords happy. Make sure they know the areas you are looking at. Request references from other investors and ask for a copy of their license.
In my opinion, you match the asset class to the manager. I see single family managers take on commercial and I cringe at how they handle the properties. I have never seen a commercial manager try to take on SFR's, but if they did, I would think the results would be equally bad. Multifamily +50 units is a specialty all on its own. There is a lot of overlap in the smaller multi's and single families, though.