Did a fully supported tour in '07.

Pro's:
No thinking req'd- just get on bike and follow the route, stay in the hotel they planned and eat where they say.
Ride a road bike (lighter, faster, more fun, less stuff to break).
Somebody's there to help fix things (including you) or to sag your tired butt.

Con's:
No thinking req'd (I mean part of the fun is figuring stuff out, right?)
EXPENSIVE AS HELL. Doing it yourself will save you probably half and you'll eat better and stay in nicer digs. Camp or do Warm Showers and you'll cut that cost by at least half.
Tour group will have a very fixed schedule and you WILL NOT DEVIATE FROM IT!!!! You'll feel rushed. The daily mileage is FIXED...today will be 37 miles and tomorrow will be 120...shut up and do it.