I've done the illuminations cruise twice. The first time was 11 years ago the night of our wedding. We booked 2 boats, and back then we had to call on the exact AM reservations opened up to get the boats. We had fun, but for some reason they didn't decorate the boats even though we did tell them it was our wedding reception, essentially. The other boats were decorated for birthdays. Tha'ts just an aside, I really didn't care, it just would have been a nice touch. (actualy, I guess I must care, since I mentioned it 11 yrs later!!

)
Then we did it again in October last year. That time I called and booked it on a whim a few weeks before out trip, and had no problem getting a boat. I don't know why, if it's priced itself out of contention or I just got lucky. But not too many boats went out that night.
It's very enjoyable. They take you on a cruise of the waterways around The Swan/Dolphin/YC/BC/BW, up to Hollywood studios, and then dock under the bridge by the international gateway at Epcot. It's a great view of Illuminations, particularly if you get the first row of boats under the bridge. They had drinks and snacks available for the regular price of the boat last year, which was an addition from 2003.
I would say we really enjoyed it both times, but didn't have our socks knocked off (if you know what i mean). Was it worth it? I think if you have to scrimp and save for your disney trip, this is a lot of money and you could spend it elsewhere and get more bang for your buck. If you've been to Disney a lot and never did this, if you just loooove illuminations and want a great view, if you have to have a great view and are there in a busy season and don't want to hold a spot for a long time, or if you just plain old don't have to worry about your vacation budget then definately go for it!
I hope this helped
Oh, having IPO book it. Sure, I would. We didn't because we decided for some random reason at home when planning the trip, and I just picked up the phone and called. 11 yrs ago we didn't because, well, we weren't staying CL, had no idea CL existed, and wouldn't have been able to imagine ever being able to stay CL

I guess I should stop and think how lucky I really am every time I grumble about little things during the day or throughout life