It inspired Win95. It inspired the original Netscape browser's UI (toolbar etc is very NeXTSTEP-inspired, not surprising because it was developed on a NeXT box). It inspired the Mac OS X and the iPhone, although both have degraded since then. As things exist today, NeXTSTEP from 40 years ago (!) looks much nicer - and way more usable - than today's flat UIs.

Being the inspiration for a bunch of better products don't mean that it was inherently good itself.