In GCC 14, C++ modules were unusable (incomplete, full of bugs, no std modules, etc). I haven't tried 15 yet but if that changed, then it definitely qualifies for a "great improvement".
Still no std modules but otherwise likely useable. modules are ready for early adoptors to use and start writing the books on what you should do. (Not how to do it, those books are mostly written though not in print. How hou should as is was imbort std a good idea or shoule containers and algorithms been split - or maybe something I haven't though of)