I like it, many times products on AppSumo are very pretty and polished but aren't very useful and buggy. This one is different in that it's kinda ugly (in an endearing, looks like an engineer designed it kind of way) but it's is very useful and functional!

My two suggestions:

1) I think if apispreadsheets hired a designer and made the website and ui prettier and think it'll probably attract more customers (I hope the company has a long life so it doesn't shut down).

2) One thing I wish these api type services could use is some kind of caching layer that users can control so that I can indicate to have a sheet cached, set the expire time, or clear the cache. And if the api is retrieving from a cache GETs should be theoretically much faster and appropriate for larger sites. And maybe cached requests could even count towards less requests since it doesn't have to talk to the google sheets api all the time.

Thanks