I guess that depends on your personal feelings towards Illuminations.
Personally, I love Epcot, love Illuminations, and I've taken the cruise a couple times and loved it both times. One time we were able to get right out past the bridge and had a crystal clear view of the show, and it was way more intense than when you just watch from the railing. I think it's a fabulous experience (especially if you get a nice bottle of wine or two, and a cheese tray or something to enjoy during the event).
Generally, the captain will drive your party on a bit of a pre and post cruise tour around the lake, all the way over to Hollywood Studios and then out to Epcot to view the show. I didn't clock the trip, but it was more than an hour last time we went, for sure.
Note: If you have an Annual Pass, you can get a pretty good discount off the cruise.