We've done it for our last two trips. Here are a few things we learned...
1. They will 15 hold passengers, but not 15 PLUS luggage. We had 11 people with luggage, and it was pretty full.
2. Personal car insurance may not provide coverage on anything with a seating capacity of more than 7 (minivan). Call your agent first.
3. Book early. Vans RARELY get special last minute rates. In fact, they can get really expensive within a couple weeks of your trip.
4. Two one-way rentals might be cheaper, BUT call to make sure the second location will even rent it out. Alamo let us take one from Sanford to Dolphin, but Dolphin wouldn't rent one when we left. We rented two full size cars, instead. Flying in and out of Clearwater, it was almost the same price to keep the van for the entire week.