It seems to me that KJVonlyism has just made people want to read the KJV less. It's a shame really, it does have a certain something to it.
I'm no biblical or language scholar, so I won't comment on any deep subjects on the translation itself, and I won't try and make the 'only the KJV translators were inspired' argument (because I think it's nonsense), but it does serve some advantages over some other translations.
Firstly, and this is probably just an advantage for me and people like me. I read fast, really fast, give me a good paper back, and I can finish it in a couple days, with only a couple train journeys and a few hours in an evening, however, I will remember very little of it. The awkwardness of the olde english forces me to sloooow down when reading, and I find I remember more. This is also true when we switch translations now and then, helps to keep things fresh and maybe see some other perspectives.
Secondly, it's public domain (although the crown still hold's copyright on it.) I can download the KJV text and do whatever I wish with it (which I have done, see ereaderbibles.com). However, there are other public domain bible texts out there (ASV, BBE, WEB, etc.), so this doesn't make the KJV unique in that respect.
Thirdly, you can sound reaaaaaaaaaaaally holy when you quote it.
However, if I'm preparing a class for my small group or Sunday school, KJV is not such a good idea, although I'm sure there's people out there that would disagree with me.
However, it does disappoint me when I think of the things that we as Christians can use use to divide us, surely our favourite bible translation shouldn't be one of them, right?