I just returned from BlogOrlando. I actually choose it over BlogWorld Expo because the sessions sounded more interesting. I made a great choice, the sessions were fantastic.
BlogOrlando was the third unconference I attended this year. Actually BlogOrlando and DrupalCamp Colorado were a modified unconference format.
A traditional unconference is based on Open Space Technology. The basic premise is there is more expertise in the audience than on the stage at most conferences. Like the unconference the day before Office 2.0, the participants show up and decide the agenda.
BlogOrlando and DrupalCamp had agendas that were developed ahead of time by their organizing committees, that's the modification to the traditional unconference format. It works really well. You get the wisdom of crowds (sessions moderated by volunteers, but with a lot of audience participation).
They are also ruled by the "law of two feet." It states if you aren't contributing to, or getting something out of, a session, get up and go to another session.
These unconferences' other huge advantage is they are organized by a group of passionate volunteers and the audience comes prepared to participate, two elements missing from commercial conferences.
I still attend a lot of commercial conferences, many are well done, but I will state the return on time (and clearly money) invested for the unconferences has been higher.