I haven't used a PM company so I don't have any direct experience with them but I would use one If I could find one that was good and charged a fair fixed price. I have seen several properties run by PM's and they are more often then not a big mess. Tenants have trashed the place, repairs have not been done in years, and even the units are empty and the manager didn't even know it. Now some of this could be because the owners didn't want to put any money into repairs, so it might not all be the PM's fault but it doesn't give a good impression of the PM either.
My wish list would be:
1. Regular inspections, preventive maintenance, and scheduling of large repairs when needed. Basically keep the house in the same condition it was when It was handed over to them.
2. A set 10% fee. and that is it. No leasing fee, no repairs fee, no eviction fee (after all it is your fault you didn't screen the tenant properly), no marketing fee, no empty unit fee. etc. It should just be a 10% of gross rents collected and any actual costs such as repairs, lawyer fees, utilities etc should be recorded online with a picture of the bill to prove it.
It just bugs me that someone charges a percentage of rents and you have to pay them again anytime they do any work.