Using compress=zstd on system root saves me about 35-40%. And I save about 1/2 the space if I use compress-force=zstd mount option. I do this on computers with SDHC/SDXC cards which have dog slow writes and lots of spare CPU time, and some of the time on HDD. I don't use compression on computers with NVMe, it doesn't appear to help much at all, and also I'm pretty sure the compression is currently a single threaded pipe, so for multi-queue devices it's plausible some workloads could become slower.

Another use case, soonish I hope to see support for selectable compression levels for zstd (currently only with zlib). And also zstd supports use of a training file for even higher ratios. Both would work well for archiving and seed/sprout use cases, where taking the heavy front end hit with slow writes will be worth it (faster download times, and of course the sprout can have a different compression level or no compression so subsequent writes can be fast.

Latest squashfs support includes zstd and also selectable levels, if you need a read-only image.