We decided against the VeniceCard, but we did get the 72-hour vaporetto pass. For us, it was a no-brainer. You need only 5 trips for the 72-hour pass to pay off (a single fare is 6.50, and a 72-hour pass is 31 Euros). We used 2 just to get to/from our hotel when we arrived in/left town, 1 for Rick's Grand Canal Audio Tour, and we spent a day island hopping (1 to San Michelle, 1 to Murano, 1 to Burano, 1 back to Venice). And, while we walked a lot, I think that we used it a couple of other times as well.
For us, the added expense of the VeniceCard was not worth it.