Derived from Hip-Hop. Pioneered by Massive Attack. Influenced many artists of all genres (including hip-hop). People like Tricky (now based in NYC) work with artists of all sorts (Jay Z, Gravediggaz, Mary J Blige to name but three)...

Beats and tunes are usually more dense and layered than hip-hop. Which are relied on much more than singing/emceeing. Which instead are used sparingly. Pushed forward and under.

It's not all that widespread. And well, that isn't really the 'point' of it. It's a genre (sub-genre?) that is meant to remain on the outskirts, occasoinaly moving into the centre with such albums as Mezzanine, Blue Lines, Maxinquaye etc.

Anyone listen to trip-hop? What do you think of it?

Don't compare it to hip-hop and put it down. Because that'd be ridiculous and pointless. Reasons stated above.