Well it was a guy at my local LFS who told me they had a big fat Mandarin in their Acro tank that went thinner and thinner and eventually died after about a year,,, I've read similar things online as well.
Mind you, this is the same tit that tried to sell me a mandarin for my 10g Nano when I was a NewB (oooh,, yeh, they eat flake,, it'll be fine).
http://www.wetwebmedia.com/mandarins.htm is a great little article about Mandarins that scared me off Dragonets permanently.
I do have 2 Scooter Dragonets, one in a 75 and one in a 90 that I bought nearly a year ago because they were labelled 'Blennies' ,, i never realized they were dragonets (should have researched that better, I know),,, they seem to be doing fine as both tanks have buzzing refugiums and they don't have much competition for pods.
I think the biggest problem with Mandarins is they are both staggeringly beautiful and amazingly cheap,, I've seen them online for less than $12,, I think it's just too tempting for some folk.