I'll have to go back and listen again to my remastered copy of PS to say for sure, but my recollection is that it's mono. I agree with LG that in general, stereo listening is superior to mono, but the only version of PS I have that claims to be stereo is the LP that was packaged in a three-disc set back in the late '60s called The Beach Boys Deluxe Set. If you read the fine print on the box cover it says it was "electronically rerecorded to simulate stereo" or some such crap. What they did was simply double-track the recording with one channel having the lows rolled off severly, and with a micro-second delay introduced into it. It's far, FAR from a satisfying listening experience.
I'm bothered by the fact that I can't recall if I have a genuine stereo version or not (I'm at work right now), especially since I'm a fan and one of those who will tell anyone who'll listen that Brian Wilson is a musical genius. I think PS is one of the greatest albums of all time, though I have my own personal doubts as to whether it's the absolute best, or even 2nd best. But I feel the need to say in defense of the album that it gets short-shrift from a number of critics and listeners because of the lyric content of most of the songs. It suffers from being on the tail-end of the "love song" era of serious pop/rock music, when people were beginning to expect the subject of songs to be something a bit more weighty than adolescent yearning and angst (a paradigm shift made most obvious in the Beatles' stylistic jump from Rubber Soul to Revolver). I think if one listens to the music on the album, and discounts the sometimes lightweight lyrics (even though they're exceptionally well written for what they are), one can gain a greater appreciation for the album as a whole, when taken in the context of the times. There's a very good reason, IMO, for Wilson to have referred to that record as a collection of little "pocket symphonies."
Cause like I said earlier it's all interpreted! 
