I did the Venice pre-trip in April 2012 and all 16 of us did the Gondola and dinner. I loved both. We were invited to sing along with our musicians,and people on the bridges and walking the streets beside the canals gave us a big round of applause as we floated by. It didn't matter that most of us didn't speak much Italian...the musicians coached us on the words, esp. the chorus. Our meal was excellent although we were in a private room that was rather "cozy"...read smallish.
Worth the cost is a personal decision, however, I couldn't imagine being in Venice and not doing this activity.
If you book your own...don't use the gondolas beside the Grand Canal...walk into the neighborhoods behind San Marco where you can book a ride at a better price and enjoy the romance of the intimate canals winding through the pretty neighborhoods.