I agree whole heartedly that discus need good, clean, warm water. But I highly disagree that they need a low pH to thrive. My own facility has run at a pH of 8.0-8.3 (depending on time of year) and I can easily keep and raise discus in this water. I know of a few other breeders in the US who have a pH a little higher than mine with similar results. As its been said earlier in this thread, consistency is key not necessarily the actual pH level (same goes for temp). Several of my suppliers in Singapore have a pH in the high 7's and keep discus with no problem at all. The ONLY time you need to give a big concern over pH is if it is into the high 8's or if you are trying to spawn them (for spawning purposes, a low pH below 7 is ideal). Beyond that, they do great at almost any level. Water changes IMO are dependant on the water parameters, stocking density, filtration, and types of feed being fed. A tank heavily stocked, fed beefheart daily and relies only on sponge filters will indeed need more frequent water changes than a tank with 1 fish per 10 gallons, wet/dry filtration and a good quality flake/pellet food. Frequency of water changes also depends highly on what your goals of discus keeping entails. If you're raising future breeding stock, then yes daily water changes are highly recommended. If you just like them because of their colors and whatnot and have no intention beyond that, then water changes less frequently is appropriate.
As far as if they are hardy or not....I will say this. Certainly not as hardy as a goldfish, but a hell of a lot hardier than most books give them credit for, especially tank raised ones. But then again, how hardy a discus is really depends on where you get it from. Get it from a LFS and chances are they wont do too well. Get discus from a reliable and reputable importer/breeder and chances are they will do A LOT better. That I know for a fact. Wild discus tend to be a bit more sensitive, but really not to a large degree. There is a markable difference in a good quality, HEALTHY discus compared to a common imported cheap discus (found in a LFS or from a bad exporter). Most common cheap imports come in hormoned or heavily colored which compromises their immune system...which allows any parasite or opportunistic bacteria to invade and get the fish sick after its imported (every fish has some parasite or opportunistic bacteria on/in them...a good quality HEALTHY fish can easily fend them off and not be bothered by them). These fish are commonly found in your LFS as they are far cheaper than a better quality fish. Most people get these because they are readily available. They end up dead sooner or later because of reasons listed above (or just from poor husbandry due to the keeper). So again, how hardy a discus is really depends on the source of the fish. The same holds true for just about every fish out there.
-Ryan