Now it really depends on if you are the one arranging it or just attending. It'l be a cool fun trip for people who just attending the wedding. But it could be tricky for someone is in charge of the arrangements. I had attended one wedding and it was FANTASTIC. Very well managed. I'm it was a memorable time for everyone who attended the wedding.The cruise members had special meal, special programs for the guest which was awesome. It was wonderful. Check this article which could help someone choose the cruises lines for weddings.
http://www.cruisecritic.com/articles.cfm?ID=4
If you ever get a chance to attend one, def attend it and let us know your experience.