Let's see - the level creation tools for the first incarnation of id's Doom
were built on a NeXT, as was Berners-Lee first web server and browser. The
interface for Mathematica was designed on NeXT as well. Modern day MacOS X
is heavily influenced by NeXTStep and it's Objective-C development
environment, as is iOS. WebObjects was a fairly popular application
framework in the late 90's and early 2000's - I know of a few major websites
that used to run it.
I'd say NeXT was relatively successful, in that it was used to create
popular products and had a direct impact on later products.