It looks like NeXTSTEP but it isn't really following its concepts beyond those looks. I did use NeXTSTEP for a bit some time ago, after years of using Window Maker and NeXTSTEP's window management felt very primitive in comparison.
> Nowadays it seems mostly stable, to put it on a nicer way.
Sadly it has some bugs, though it is largely external stuff (RandR support was never implemented properly and is so broken that i don't think it is even compiled in by default, mouse settings do not work since it uses some old APIs, EWMH support is buggy/incomplete and some applications that assume it is always there have issues under it), but UI-wise it is stable (in that it doesn't try to reinvent itself - a 1997 screenshot of WM looks pretty much the same as a screenshot from a current PC, aside from increased resolutions, at least unless you are on a cheap laptop :-P).
I do try to fix some bugs myself and implement features whenever i get too annoyed by something though.
It was actually my window manager of choice between 1998 and 2003 when on Linux distributions, and I had my collection of mini widget apps.
Nowadays it seems mostly stable, to put it on a nicer way.