Well, I am about to buy Aphex Twins SAW2, for the second time. I bought it first pretty much when it had just come out, on audio tape. A friend and I sat down to listen to it on his dads hifi in an acoustically treated room, and we were amazed, we just sat there chilling out for the whole album, letting our minds wander with the highs and lows of the moods it gives you. Not once did we think of stopping it, in fact we barely spoke for the whole of it. Obviously we had to turn the tape over, but my friend was sitting next to the deck so it didnt intrude too much on our ascension.
However before buying this understand it isnt ambient music in the way you normally imagine (think Orbs early stuff) but is a state of mind instead. It isnt like tranquility music featuring whales and dolphins etc, its mostly electro stuff, although im sure some of it is sounds from objects mucked around. But it doesnt have beat patterns or anything like that. Its designed to allow your mind to ponder, and is especially good at inducing alternative dreams (nice, odd, or in one case terrifying!)if listened to late at night.
On the whole, you cannot go wrong if you have an open mind toward sound, not just towards music.