Both 3rd edition and 4th edition put way more effort into making Gnolls playable than they did for the far more popular (and less difficult to justify) Goblin race. So obviously someone at WotC has an agenda in regards to them.
That being said... I do think Gnolls are meant to be demonic, low intelligence humanoids that are more beast than man. Having them become common heroes is a bit odd in the way things are set up in D&D. I would find it far more acceptable in, say, WarCraft, where their origins have nothing to do with demons. And, obviously, it is entirely possible to have a world using the D&D system where their origin and motivations are far different. There is nothing about large, burly hyena-people that indicates that in all possible realities they must be demon-spawn. You could very well have a world where some wild magical force caused the plants and animals in an area to undergo wild mutation and Gnolls were a result of that-- or maybe they were created by a mad wizard as some sort of shock trooper and the mad wizard ultimately died leaving them aimless and rudderless, just trying to survive in any way they can,which generally means sticking together in packs and killing and eating anything or anyone they come across. You can then more justify that typical member of the species would be considered "evil" by humans and the ilk who resent being hunted and having their crops eaten and their homes taken or destroyed (even though it might be more fair to say they are "neutral" since though they commit theft and murder on a regular basis, there is no malice behind it just as humans who hunt for sport and animal predators are not considered 'evil'), but there is no reason a couple couldn't be lead from the default predator/scavenger mode with good influence.
Even with demon worshiping Gnolls, I suppose it is possible that one might find themselves exiled from their pack, disillusioned with their religion and willing to explore other ways of living. Though it is highly doubtful that any particularly civilized race would give them much of a chance. Such ones would probably end up serving Hobgoblins or Dragons or something.
And since people want a furry animal people (not even debatable given how Dragonborn were shoe-horned into every setting ever and there are hundreds of cat-people and dog-people races that people constantly try to get over), there is little doubt that certain players are going to be drawn to playing Gnolls.
Even if Gnolls are 100% Chaotic Evil (or at least within 1 alignment step of that) and below average human intelligence and civility, I don't know that one could really say that such a thing is "unplayable". It just doesn't mesh with any of the standard adventure types that make it into print.
All in all, I have to say that naturally Gnolls are something that is playable. Its just that they are probably the most difficult one to justify if you are playing in a very canon GreyHawk or Forgotten Realms. I think they would probably be fine in Eberron though.