It depends on personal preference and how much you are willing to put up with. Also as you get more and more units it will probably become a necessity to hire some form of management.
I would look at what other fees they charge outside of their monthly management fee. For Example, check writing fee, bank fees, cleaning fees, landscaping, snow removal, etc. Also, Are they going to Rent the units for you by putting a tenant when vacant.?
I concur with my buddy, Matthew Teifke. Its opportunity cost. Its best to turn it over to someone that can handle those affairs efficiently and effectively as they have the mechanisms in place already. The learning curve can be expensive if you don't know the rules of the road such as good screening of tenants, effectively dealing with unforeseen issues such as pest control, etc then it could be tough. I have learned that its best to put the players to play and if they don't perform well get a new player. I like basketball alot so I will use some related analogies. If you buy a tough property with a multitude of issues and you have very little experience it would be like putting Spud Webb in to guard Shaq "Attack" ONeal. Guess who is going to be dunked on... Also keep in mind that folks like Bill Gates became great by hiring the right people and/or firing when necessary.