I've used several PM systems to automate my vacation rentals. OwnerRez is by far the best of them and I've stuck with it for the past 3 years. When you look at what you get for the money I can't see why you'd use any other product. And the CS is by far the best. All of my questions have been answered in just a couple of hours. I've even gotten answers over the weekend. Highly recommend.
The API integration to several OTA's, the communication automation, the customer support, the reporting functionality
The WordPress widget could support broader integration between the system and your website (e.g. the widget doesn't push new/changed photos, descriptions, etc to the website).