I ordered three Idols through a friend, thinking they may do better in a group, when I was setting up my 240 gallon tank. When they came in (Jan 22, 2009), my tank was not ready yet, so my friend held them in a sump of one of his tanks for about a month or two.
Well, being in a small area in a sump, two of the Idols ganged up on the other and picked on it til it died. Then the other two battled til there was only one left. So I have that one, I never took possession of the other 2 (or even saw them, the above pic was sent to me from my friend). I don't know if they would have been OK if they went into the 240 rather than in the close quarters of a sump, but I suspect they may have done better.
At any rate, I have had this one in my tank since March, so about 8 months now. I really love this fish, it is always swimming around, always looking out into the room and watching everything that goes on. It eats well, and I feed it mostly pellets, but also frozen foods including mysis and Rods food. Early on, it stopped eating for a day or two a couple of times, but I was always able to get it eating again with Rods food. However, the last 6 months, feeding has not been an issue. It is really a wimpy fish with no real defenses that I can see, so you can't keep it with anything aggressive. I have it with an Asfur Angel, a Yellow Coris Wrasse, a rabbitfish, a Firefish, 3 Blue Devils and a Royal Gramma. The Asfur Angel chases the Moorish Idol unmercifully, but the Idol just takes it in stride. Another thing is the Moorish Idol has developed a taste for zoas and palys, and has pretty much wiped out all my zoas and palys. Otherwise, it has not bothered any of my other corals.
Also, it came down with a mild case of ich a few weeks ago. It looks to be mostly gone now, but it still has a few spots. It never stopped eating the whole time.
So, to-date I am extremely happy with this fish, it has been great. I hope it continues to thrive as it has so far. I don't want to recommend it for everyone to go get one, but it has really far exceeded my expectations so far. By the way, I have been keeping saltwater fish for over 35 years now, so don't think I am some newbee guy recklessly trying my hand at such a difficult fish...