"Serviceable" is probably the best way to describe Zeffirino. In a city stuffed with superior Italian restaurants, Zefferino falls squarely in the 'mediocre' category. That is not to say that it is poor by any means but at its price point and considering that it's in the Venetian, you should be expecting a lot more.
Pros:
* $22 prix-fixe for lunch is a great deal
* the pasta dishes in general are fine
* the view of the Gondolas does lend itself to a romantic occasion
* the musicians at the restaurant are very versatile and will perform songs in multiple languages
Cons:
* the food is simply not spectacular and is easily outclassed by nearby restaurants such as Valentino, Postrio, and B and even Canaletto
* the prices are ridiculous. $28 for foie gras is more expensive than in top end steakhouse and only $2 cheaper than the infinitely classier Le Cirque; $20 cold cuts when you can get the exact same dish for 1/2 the price 5 minutes away at Enotecca Otto; $44 for a simple breaded veal dish that you can get for $20 in any neighboring Italian restaurant, etc....
* exceedingly small portions at the limited dinner prix-fixe (it costs the same as a single entree for good reason)
There is better Italian to be found in the Venetian and in Vegas in general. The ambiance is pleasant and the musicians surprised me when they knew Croatian and Serbian folk songs but on the basis of food and service and especially value I simply cannot recommend Zefferino. An a la carte dinner with appetizer, desert, and a drink can easily cost $100, the same as the tasting menu at a great restaurant sage as SAGE or a fantastic steak dinner at any of the multitude of Sin City's red meat havens.