Just getting into Bonsai and did pretty extensive research what tools to get started with. Reading everything available to me online I found Kaneshin tools are trusted for their quality and performance, so that was a good starting place. In the three trees I've gotten to use my Concave Cutter I've thoroughly enjoyed it. Not to say that I'm not "bootstrapping" with other non-bonsai brands/tools, but as I get more and more into it I look forward to adding more Kanshin tools to my arsenal.
As for the specific tool, I opted for the (804) Round Edge Stainless Concave Cutter. I knew the primary/immediate use was to be for my Azalea projects, so taking advice from Kaneshin's website I went for the stainless cutters, opting for easier maintenance in place of the sharper blue steel options that require more upkeep. In the future I'll invest in some blue steel tools, but I want to be more familiar with cleaning and sharpening practices so for my first tool I went stainless. The cuts are clean and effortless, and I'm happy with my choice.