I kept one for a year or so in a reef. I only bought him cause I saw him eat frozen at the lfs. He acted alot like a tang and ate about the same diet. Over time he'd eat anything, including nori off a clip. I fed him 3 times/day and never had an issue with him nipping corals, only a clam mantle every once in awhile. I lost him in an ich outbreak that came in on some live food (glass shrimps). I fully plan to get a pair of them later this year for my 475 reef.
They are not impossible to keep, as long as they'll eat and acclimate without going on hungerstrike. I know of a fish dealer in Dallas who personaly has one in a very large reef now, and has had him for 6 months or so. I'd NOT try one in anything under 120gal, or a crowded tank or an agressive tank. Idols seem to be very placid - like calm tangs, and prefer IME a docile tank without too much competition for food/space.
I worked hard at offering mine a VERY diverse diet and fed 8-10 different frozen preperations, various color nori sheet, dry foods, and the occasional live foods. (it's the live food from LFS that inroduced ich and the idol went on hungerstrike in qtine)
I'd still have him today if he'd survived qtine, casue that was the
last time ANYTHING from another system went into a DT of mine without some effort to Qtine out pests.