It's... I dunno, it's like FVWM95, only from this century. I used Openbox + a bunch of tools cobbled together before. It doesn't do anything my old setup didn't do, but it sure is more comfortable. No scripts, no custom setups... all that was fun twenty years ago but I'm not a teenage l33t h4x0r anymore, I got work to do nowadays...
I'm not sure why people conflate it with Lubuntu, you can use it on any distribution, and the fact that it's very easy to package means there are few packaging-related problems with it. It's also pretty easy to compile from source if you need that, for whatever reason.