I think it's worth it, at least once. As a youth I chose the National OA Conference over Jambo (showing my age - that only happened in '81) and didn't regret it. In '05 I was 1st ASM for my council troop and had a great time. My Webelo son is pumped because he will be 13 in '10 and can go.
The event is unique - check out the scouting.org site for a new preview video. All in all, I've personally had better/cheaper Scouting experiences backpacking in Colorado, skiing in Austria, scuba diving in Texas, canoeing in Arkansas. But Jambo is unique and I want to do it one more time with my son.
As for the weather, some Jambo's have been nothing but rain, the last one had unrelenting heat until the moment the opening show was supposed to start, next time it might be relatively cool. The key is to BE PREPARED.
Folks talked about the death march. Yes, it was a bad, dangerous decision to go forward with the march, especially since the show was likely to be cancelled anyway due to the front and lightning storm coming.
But our troop was prepared. The leaders bought hot dogs, chips and powerade for the youth and made eveyone eat before 3 PM. We carried all the water and electrolytes we could with us, knowing it would be thrown away before we entered. During the 3 hour hike we scavenged all the water we could and sang to keep up morale. The only problem any of our guys had was desperately needing to use the bathroom by the time we reached the show site.
The health and safety issues of Jamboree are your biggest concerns. Having a second asst. SM with military medical training was priceless. He pushed hydration PLUS electrolytes.
Plan, prepare, and train and it will be worth it.