It's cumbersome to use, it takes even longer to load than AdWords itself. In the end, some steps and processes are supposed to be faster (like adgroup splits) but if you're really agile with AdWords and excel, I'm not sure who would win in a head to head race.
It doesn't understand the central keywords of your advertisers. If you sell "bed sheets" it keeps on insisting that you eliminate or separate "sheets" from "bed". The tool that supposedly allows you to tell the robot not to do that doesn't work, because it repeats the recommendation next time.
Wordstream insists that the search queries' language doesn't affect the robot since it is data drive, we have several european clients and we know that is not true. We can tell the recommendations are built around english language structure.
Besides these basic flaws, Wordstream is unable to help you when: conversions are more complex because they involve phonecalls, or you have accounts that work with converted clicks and other with conversions, or you want to optimize through microconversions, or you want to work with ROAS (revenue/cost). It doesn't help you adjust for location, day time or even mobile bidding. It doesnt' help you optimize extensions such as sitelinks or callouts.
Finally, their customer service is extremely poor. They have a very good blog and exterior communication, and they tell you they'll have an account adviser for you who will give you specific recommendations. Working with them nearly 2 months we didn't get a single report or advice. After a request for advanced insight on adwords strategy, we were answered with a predictable template that purposefully misunderstood so that the AM could get rid of the task easily. Upon futher insisitance, we never got an answer. A week later the relationship was severed, even though we still have to pay minimum mandatory subscription for another 4 months. Review collected by and hosted on G2.com.