We've been on bunches of them, and in retrospect, it comes down to one very basic issue:
What does this liveaboard offer over and above land based options ?
This is why a Bay Islands boat must burn a whole lot of fuel (move the boat great distances) to make the price worth it. This is why they do (and will) fail... note the Grenada boat that never went to the best (by far) diving on the NE end. A liveaboard cannot charge $3500 when you can get the same exposure for $1200 land based.
Liveaboards are an excellent way to see, in example, the Out Islands of the Bahamas, the Maldives, and Belize. There is really no other way to see the diversity of these places.
You start to get in a cost benefit ratio quandry in locations such as Truk, the Philippines (that's Fiji on a budget) and the Galapagos. It is easier, simpler and waaaay cheaper to do land based and get the same net result. There will be people who disagree, but few have tried both the ships as well as the land based operation.
I think the best way to do the GBR is liveboard. The Solmar V offers a unique product if you can stomach the crossing.
RobinT I believe that this is certainly not your first Nekton trip that shows in your "next trip countdown" thingie. I have several trips aboard the Nekton fleet and you will likely use them as a standard by which you measure all other liveaboards. I would highly recommend their Southern Bahamas trip. Saw many Sharks off of San salvador. Real Sharks.
A good place to see this dichotomy in action is the Red Sea. I will guess that day trip divers ratio to week long liveabaoards would be 1000:1, yet we Americans still must have the liveaboards and miss the fun and cultural excitement of Sharm el Sheik at night. Baffles me. A harder place to see this is in the Galapagos. Harder because most people have not done both so they don't know what the land based has to offer.
On a side note, I have seen plenty of liveaboards fail because of reasons that were not apparent. Difficult restrictions by the local government (pressure from local operators), difficulty in precise, reliable or timely air connections and resupply. I went to the Antares III (Los Roques) repeatedly even though it had an inherent flaw in its sewage system- it took me to wrecks that no one else saw. It was either the stink or the shallow dives with swirling sand filled waters that scared most away.
You're young... do them all!
Of course, the list will probably change a few times between now and then, too. Oh well, gotta dream! 
