I have done the Breathless Illuminatiosn Cruise 2x - Once in 1998 & once in 2001. In 1998, I felt it was worth it, but more for the post-fireworks activities than anything. It used to include a lengthy high-speed, acrobatic "Burst" throughout the lakes & canals from IG to MGM. We loved that part so much, we booked for another special occasion in 2001.
Because of legal issues, no more "Bursts" at night. It was definately NOT worth the money AT ALL. It is really about the only time I have ever felt completely ripped off at WDW. I was extremely disappointed & angry. As posted above, the viewing area isn't the best, you can't see that much, it's crowded with boats now & you are at water level & can get quite overwhelmed with smoke. I've had much better views of Illuminations from WS for free.
Now I watch Illuminations from WS walkways & take a Breathless Burst from Y&C marina for 20 something dollars during the day. A much better use of my time & money.