I pay mine 7% of rent - you can definitely find lower rates if you're dealing with large buildings, but I'm at 7 units total and I really like the guy.
In terms of vetting them, first thing is to check licenses - most states require them to either be a licensed real estate broker or working under one.
I prefer to work with someone who matches me in scale/style - my guy is a broker and runs his own PM service. I deal directly with him. I'd be hesitant to use a larger PM company (even if it might be a bit cheaper) at my scale, because I'm small enough that I'll be a very low priority and will have the most junior employees dealing with my properties. That can be an issue, since they're not necessarily going to have the expertise to do the long-term stuff like knowing when to get the roof inspected. When my wife moved out of her last apartment before we got married, the rep from the PM company refused to do a move-out inspection, since there was still furniture inside that could have been hiding things. Had they charged my wife for anything, we would've taken them to small claims since PMs are required by CA law to inform the tenant of their right to a move out inspection and then to perform one and provide written feedback on request. That's what happens when you're dealing with someone in their early 20s who just passed the real estate agent exam (which I have passed... it is not hard) and doesn't know what they're doing.
Also, look for companies that serve your kind of property - dealing with a property that has an HOA is different than one that doesn't. Dealing with a single family home is different than dealing with a condo. Dealing with a whole apartment building is different than dealing with a single apartment.
And lastly, references. You should be able to get a bunch of them from the PM. Try to get tenant references in addition to owner references - if the PM is ignoring or mishandling tenant issues, the owner probably doesn't know.
In terms of vetting them, first thing is to check licenses - most states require them to either be a licensed real estate broker or working under one.
I prefer to work with someone who matches me in scale/style - my guy is a broker and runs his own PM service. I deal directly with him. I'd be hesitant to use a larger PM company (even if it might be a bit cheaper) at my scale, because I'm small enough that I'll be a very low priority and will have the most junior employees dealing with my properties. That can be an issue, since they're not necessarily going to have the expertise to do the long-term stuff like knowing when to get the roof inspected. When my wife moved out of her last apartment before we got married, the rep from the PM company refused to do a move-out inspection, since there was still furniture inside that could have been hiding things. Had they charged my wife for anything, we would've taken them to small claims since PMs are required by CA law to inform the tenant of their right to a move out inspection and then to perform one and provide written feedback on request. That's what happens when you're dealing with someone in their early 20s who just passed the real estate agent exam (which I have passed... it is not hard) and doesn't know what they're doing.
Also, look for companies that serve your kind of property - dealing with a property that has an HOA is different than one that doesn't. Dealing with a single family home is different than dealing with a condo. Dealing with a whole apartment building is different than dealing with a single apartment.
And lastly, references. You should be able to get a bunch of them from the PM. Try to get tenant references in addition to owner references - if the PM is ignoring or mishandling tenant issues, the owner probably doesn't know.