For $19 it was an easy decision to buy.
I've been looking for an affordable scheduling app. The LTD lets me avoid ongoing monthly fees that more established providers rely on for their income stream. For Appsumo, they are well-established and so I'm not worried about how long the "lifetime" of this deal will be. Having seen their treatment of Sendfox, I'm sure the product will be well supported.
As a solopreneur, I got TidyCal so that my own customers and clients can schedule meetings with me easily. It looks like it will work great for me.

I'm not offended at the modest branding. I'm a +1 for cname.

The product works as expected albeit a bit sparse on the backend features and a few minor bugs here and there. I presume that the Appsumo development team will address comments and issues raised by fellow Sumolings. For that reason, I expect the product to continue in development and improve over time.
For me, this is an investment that will keep me from having to worry about scheduling with customers and clients again.