Its not built on top of linux, its a true hybrid micro kernel. Its not a windows manager on top of a linux kernel. It is its own animal.
There's a start menu of sorts, something that looks like a task manager and weird window decorations that make using the computer very difficult (maximizing, minimizing, resizing, etc.) A bit brain damaged design. But that's just a window manager anyway, no reason to switch OS for that.
I don't see the point. I think people using Haiku are in the same group of people who still use Amigas and Atari STs: nostalgics. I suppose for some it's cool to install something on a PC today that is a clone of BeOS. But other than that, it's just pointless.