I use LibreOffice fairly often (like, several hours a day, several days a week). LibreOffice is a fork of OpenOffice, and is pretty much the new OpenOffice.
I primarily use it for working on flat files. I dump a CSV file out of some ERP software, do some work on it, and then import the data back into the system. I make a lot of use of the sort and filter functions, and have only used macros for writing some convenience formulas. Usually these files have a few tens of thousands of lines, since they are basically just SQL query results dumped into spreadsheets. LibreOffice handles them fine.
I also use it for my bookkeeping.
LibreOffice, like OpenOffice, allows you to manipulate documents using a Python-UNO bridge, but I've found it far easier to just use Python libraries like csv, openpyxl and xml to parse my files, do some work on the data, and then output a new file. And once you've imported your data into a Python data structure or database, you might as well do all of your sciency stuff in Python and just spit out the results in whatever format you need.
For instance, my accountant gave me a spreadsheet decked out with formulas and fancy logic for calculating my GST subtotals. I broke it almost immediately and just decided to remove the logic entirely and do it in Python instead. So I have an XLSX spreadsheet and a "totals.py" script that parses the tables, calculates my totals, and saves a new file with the summary table populated.
In another case, I actually import my data into an SQLite database, manipulate it in Python with the help of SQLAlchemy, and then dump completely different output files in various formats, including nicely formatted reports.
Some people are absolute wizards when it comes to writing VBA scripts in Excel, but that's something I never really got into. I've always preferred Python, and writing Basic scripts in OpenOffice/LibreOffice is a pain in the neck.
Ultimately, though, these FOSS programs are Excel clones. They are better than Excel only because they are free; I'm not sure that anyone who uses Excel would actually prefer these programs.