Easy Interface - the WYSIWYG interface makes it extremely easy to build out marketing campaigns in a short amount of time. Just place your text and images and you've got a beautiful cohesive design.
Wonderful customer support - I have had very few problems with Act-On, but those I did have were resolved very fast by my customer care rep. They are very hands on with their customers and do whatever they can to help you succeed.
Integration - smooth integration with our Salesforce account allow us to build out elaborate campaigns with specific lead lists, and then add qualifying data back to customer accounts.
The only issue I have with Act-On is that when you build out a form with a list item like a checkbox or dropdown they are hard to edit later. Essentially you enter your values into boxes and if you want to delete fields or add in fields you have to pretty much start over.
The great customer service and capabilities of Act-On are more than enough to entice me to renew my account. I have used many e-marketing products before, both free and paid, and this is the most robust system I have found so far.
There really isn't any advice needed. Act-On provides an extensive library on getting started and your customer service rep is a great asset in learning your way around. All the tools are self explanatory and built for ease of use.