The user interface, which can be very confusing for new users to figure out how it works but once I did get used to it, its been great! They have helped us with our SEO needs by optimizing all aspects from product pages, blog posts, social media content etc. Their customer service has also always responded quickly when we need help or clarification about what they are doing specifically for us. We were using them as an agency so their services were provided through another vendor who had previously outsourced work to these guys. So no direct communication between myself or my team at this point. Our previous provider was not providing full support of both Google Adwords & Bing (we use those two platforms). This made things difficult since there's overlap in some areas of each program. But overall good experience working w/them.