We saw a launch from WDW last December and it was awesome. It was a night launch and we saw it from the dock at the Poly. The night sky really lit up!
On a sadder note we got in for the 2003 Member cruise right after Columbia blew up. My DH had said how cool it would be to get in and see the landing but we already had our airline tickets and would be landing at MCO about 10-15 minutes after landing. We got off the plane, called FIL to tell him we were safe and got the news. It was a very sad ride over to the port listening to the coverage of the event.
As for how many will be on the ship, as Cheryl stated it will pretty much be a sell out cabin wise. However the Member Cruises tend to have a lot fewer children on board than regular cruises. So the ship is a little emptier. Instead of cabins filled with 4 (2 adults, 2 kids) you have a lot of them filled iwth only 2 adults. This was really evident on the first Member Cruise. We had late seating (which was the first and only time we tried it) and the dining room was just over half full. Lots of empty tables, even though all the cabins were full.