Generally, charities providing destination marathon entries will have a minimum donation amount that you will need to meet. The fundraising can range from $3-5k depending on the charity, distance from the race and accommodations. In trade for the fundraising, the charity will usually provide entry, airfare, room, race shirt and a few leisure shirts and a banquet meal. Accommodations that I am familiar with are in the moderate resorts but that is not absolute. Most charities will have local training groups in larger cities. If your local office is not headed to Disney in 2014, there are also virtual training groups. In the local groups, you create a comradery and will have folks you recognize on course.
The largest negative is the fundraising. Through contract you are on the hook for your goal and that can be a stress riser at the same time you are pushing the longer runs. Before I ran a charity destination marathon I figured out what my travel costs would have been and stopped stressing and heavy marketing once my balance was below what my travel budget would have been. That actually sped up some late comers and I think I cut a check for $400 near the end to cover my donation goal. A lesser negative for Disney is that you have no idea of what your bib number is or for that matter if you are really entered until late in the game. That is not a big negative but something you need to be aware of. Your charity owns your entry and most pass that information out on their schedule.
The positives are a great feeling created by the knowledge that you are making a difference. It can serve as a tool in your toolbox of work arounds that can pull you through tough spots in the race.
Note... this is based on a decade old event so a few things may have changed