Key traits of a good property management company

[J J Moody]

What are some key traits of a good property management firm? 

I already know I will want to outsource this aspect of my rental property business but haven't started vetting PM companies. For those of you have the experience, what are you looking for in a property manager? What questions are you asking in the interview process? Is there anything about the terms of the agreement that I may need to think through in my analysis (beyond the percentage they require to manage the property)?

In reading reviews for some local companies, most tenants seem to dislike the PM and the owner experience seems to be a mixed bag of good and bad reviews.