The most helpful feature for me is definitely the ESP routing feature. The ability to route traffic over multiple ESP's helped us to maximize our overall email success.
You need to know what you are doing. Because Ongage is "only" the front-end, you also need to work with other partners to be able to send your first email. I guess this is a threshold which new customers need to overcome.
Try it out and see what the product can do for you.
Ongage helped us a lot to scale our business. Before we used Ongage we really hit the limits of our ESP.
After the switch to Ongage, we were able to grow in volume drastically.
This, in combination with the ability to route traffic over multiple ESP's, helped us to increase our ROI substantially.