I also like that yours is generic -- I'm curious what other use cases might exist for an SMS based query system for sheets.
Another minor feature I added was lists so that my partner and I could keep our shopping list synchronized -- I could imagine it working well for that too
As for queries, I'd like to add queries to get the categories and query the amount left in a category. But that's more special casing - maybe it's a purist in me. I'm leaning towards letting people define a keywords area with three columns - the first is the word to trigger the command, the second is where anything after the keyword gets put (in some sanitized manner - like stripping whitespace) and the third would be the place to read the response.
I like the shopping list idea.
I haven't really taken the time to polish up some operational bits (we have really good logging/metrics where I work, and I miss it) or to market it though. So it's basically just my wife and I right now. I think the push to get something shipped (and my perfectionism) drained me, but reading this stories comments has been invigorating, and I an probably go about improving things in a more measured manner.