Okay, I'm waaay late to Porcupine Tree. I just started listening to them in the last 3 weeks. I downloaded every album and listened to them in chronological order (and then turned around and bought my favorites). I think they have some great music. The problem I have is that some of their albums are at least half prog-rock-space-jams. I don't mind some interludes or the occasional, well thought out instrumental; but when six out of ten songs is a prog-jam (Up The Downstair) it gets a bit self-indulgent. Having said that, when they are good, they are really f'ing good. So far, my favorite three are:
The Sky Moves Sideways
Signify
Fear of a Blank Planet
Porcupine Tree
Threaded Mode