When I bought in ( 1993 ) I did so prepared that there must be a catch, as it seemed too good a deal. I haven't found one as yet. No hidden costs, not really any "disadvantages" but maybe there are ways that with a little flexibility one can make use of to help make your points go as far as possible. I understand some people just want to get into their rooms and not move about, but if you are prepared to do so, or would like to visit other places ( USF, BG Tampa or the beaches) you can fit those things into weekends when DVC has it's highest costs. IMHO if you can give up staying on DVC at weekends those points will buy you 5 nights accomodation (Sun-thurs) later int hat trip or on another trip.
As Blue, points out the weekends are expensive. I've worked around this a number of differnet ways. Many people fly out on a Saturday ( arriving late in to Orlando), by booking a hotel near the airport for that night ( all I can do usually is quick eat and sleep) , I usually get up early the first morning anyway so checking out and moving to Disney rarely wastes any valuable part of that day. You can "pre check" into your DVC , collect tickets and make off to the parks, making your way to your room when it's ready. If you're flying back on a weekend I've found a great way to deal with the high points cost is to check into one of the Universal hotels and finish the holiday with a couple of days there. For a 3 week holiday I'd have 5 nights in DVC to start with ( maybe 1st night in a hotel as above) 9 nights in a villa ( or off site) taking in 2 weekends and finish off at WDW.