Iber's place could not be any more convenient to Piazza San Marco. We were about 30 steps from front door to the Piazza and it was as breathtaking as we expected.
The Campanile chimes every hour, for a few minutes, starting at 7am, so be prepared for that if you're a light sleeper and want to sleep past 7. It wasn't an issue for us at all and I actually used it as an alarm clock.
If you're staying at a time of year where the high tide can be expected (November, December, and October - in order of frequency), know that you'll likely need to buy some waterproof booties to be able to get to the water bus station. If you don't have anywhere you need to be and can just enjoy the experience for a couple of hours, it's really not a big deal. You can generally get to the Piazza during aqua alta, just not to the water bus station, without going through some high water.
Know that this is a 3rd floor apartment, but there is an elevator if stairs are an issue for you.
The space is fantastic and comfortable and Uber and Dario do everything they can to make your stay enjoyable, including setting up a private transfer from the airport to the apartment.
We would absolutely recommend staying at Iber's place and would love to stay again the next time we visit Venice!