I find that iWork - or at least Numbers - has a tendency to bog down my machine on High Sierra. This is true whether I use Numbers from iWork 09 or the current version. I was getting tired of that and sought some alternative.
My needs are pretty basic, so I can't speak to those who need sophisticated spreadsheet programs. TableEdit might well meet those needs, and other have mentioned here already that it can do various types of functions. There are features I haven't tried, such as charts, that I haven't played with because those aren't something I use. I'm sure there are reviews online that go through these matters in more detail.
There are some limits to it, at least at this time. For instance, I can't just drag across cells to select them. I have to use the shift key as part of the selection process. I also can't right click on a cell and have a popup of options. I either use keyboard shortcuts or the toolbar.
Nonetheless, those are minor issues for me compared to the speed, low memory footprint, and overall responsiveness of the program. It does what I need without all the bloated feeling of Numbers, Excel, etc. The price is also reasonable.
Also, their customer service is good. This version (1.4.1) has some features I requested. They did that in about 2 weeks time. I don't know if that's their norm, but I appreciate that they listened to what I said, responded to me quickly, and implemented what I needed promptly. I think they're a company that wants to grow and is quite willing to work with their clients.