They started as a solo project of the guitarrist Steven Wilson. Early PT are psychedelic/ambient/prog rock (they were called the "new" Pink Floyd), now the music they make is more experimental metal. Stupid Dream and Lightsun Bulb are 2 albums from the first period I loved: I still like almost every song.
Beginning with In Absentia they started to sound much more metal and I like that too, but I know the general public doesn't enjoy "hard" stuff.
If you are into "calm" art/prog/psychedelic rock with ambient influences I recommend you nosound (albums: A Sense Of Loss, Lightdark). Also North Atlantic Oscillation are quite good, saw them live.
Btw, just check out Kscope (http://www.kscopemusic.com/). It's label for this kind of stuff and there are some really good bands signed to them (PT were with them before signing to Roadrunner Records, a big name in the metal scene).
I also recommend you the last Pain of Salvation's album... While I don't like it at all it's a lot into the '70 rock style. And, since you wrote you are always looking for new stuff, listen to their album BE or The Perfect Element. Also Novembre's last two albums (progressiver death/doom/gothic metal... but, really, don't be scared of these labels if you don't like metal, they are enjoyable also by the average listener).