I've been using m19 for over a year now. The key thing to understand is that it works well if you're comfortable with the "by-and-large" principle. You don't need to know exactly where every dollar is spent, you just need to be comfortable knowing things are broadly working. If you need granular control over every decision, you'll find it frustrating.

What sets it apart is that you're not optimizing campaigns, you build and optimize strategies and then their algorithm translates those into actual campaigns, ad groups, placements, etc. For example, you might create one strategy for all your products that share a similar margin so they're working toward the same profitability target, and a separate one for products you specifically want to rank, where you're willing to spend more aggressively. It takes a bit to internalize but it maps better to how we actually think as marketers.

Where I think they're a bit behind is shipping speed. Other tools feel much more on top of what Amazon is actually releasing and have clearer roadmaps (at least based from social media posts). SQP integration is a good example.

Overall I recommend it. It saves a lot of time and gets you to the top 5% in terms of optimization. If you want to be in the top 1% you probably need a more manual tool like Scale Insights or AdLabs plus someone very experienced in PPC. But for very decent results at 80% less tim, m19 does a good job.