Stayed here recently for 5 nights on points and would recommend it. I have no affiliation, but the recent OMAAT trip report about the hotel did a good job covering the basics and I would generally agree with what he said. He also stayed at the Gritti, which I have not, so I can't compare directly but it does seem like if you prefer a truly historic hotel, Gritti may be the better option (though it now seems to often be going for more points than the St Regis), while the St Regis is a great choice if you want fully modern accommodations in a historic building.

I requested SNAs and cleared into a Venetian Suite at the 5-day window; I forget what the other options were but I checked all the suite options and there was at least one higher potential category, the St. Regis Suite. The hotel was showing sold out by the time I arrived so I was pleased to have an upgrade at all, and the room was nice, though not large. It was similar to the St. Regis Suite that OMAAT reviewed (including the same courtyard view) except the living room was much smaller, and the bathroom was smaller with no tub. Some hotels would probably put a junior suite in the same footprint, but it was still nice to have a door that closed off the bedroom.

The room was very nice and other than a few squeaks in the floor and other details of being in an old building, was essentially the same quality as I would expect from a new build St. Regis anywhere. We received the same welcome amenity of a bottle of pre-mixed bellini and a little cake that OMAAT reported. My only slight gripe would be that housekeeping was not quite as detail-oriented as I might expect from a property at this level, for example never replacing the small bottle of mouthwash that was provided initially, and not doing the thing you often see at 5-star hotels where they rearrange all your toiletries onto a towel (whether that is good or not may be up to personal preference).

Elite breakfast is great, with a small but very high quality buffet (including self-pour champagne and prosecco) and the option to order off the menu, and outdoor seating overlooking the Grand Canal. Currently runs until 11am.

Gym is small but decent for a destination like Venice, with a good selection of cardio machines and some basic weight training options including dumbbells, a cable crossover station, two benches, and some other accessories. It didn't seem to get a ton of use in the mornings, at least during my stay.

Location is very central ... if anything perhaps too central, with very touristy areas nearby. Many of the recommended restaurants we heard about and tried were a bit of a walk, as most of the nearby options are either tourist traps or very high end. (We really enjoyed Anice Stellato, by a quiet canal clear across town; plan your restaurants in advance as many of the good options require advance reservations.) Nothing is too far in Venice, though the streets being mostly narrow, mazelike alleyways can make it a bit confusing to get around. The water bus is also a decent option -- often not actually faster, if you're going anywhere with a walking option, but at least saves your feet a bit of damage. Remember that for now FFP2/N95 masks are required on board the water buses, and it is enforced. I would consider Gritti's location pretty much interchangeable -- Gritti is right at a water bus stop unlike the St. Regis being between two, but of the two stops near the St. Regis, the one at Gritti is the less useful one serving only a single line. (Danieli is a bit farther but also functionally in the same area, though it sounds like that property is leaving Bonvoy soon.)

Also, FYI, we were advised that if you want to go to the popular other islands like Murano and Burano, it's best to go on weekdays as the water buses can be overcrowded on the weekends and don't run very frequently, especially to Burano. We didn't end up trying, so can't confirm.

In general, it seems a lot of people do a shorter stay in Venice, and I probably wouldn't have done 5 nights if not for the 5th night free on points, but we definitely did not feel like it was too much time in the end. If you have little interest in art, there's probably not much reason to stay that long, but if you do, you could easily stay much longer -- the Biennale ended up taking us two full afternoons, and we're relatively fast museum-goers.

The hotel gave a good comprehensive list of airport transfer options upon request, from high-end luxury transfer to public transit, which I appreciate from a hotel like this. We went for the prebooked mid-range private boat transfer, which was around EUR120 from the airport and then EUR70 to the train station on departure, plus tip. Not cheap, but IMO fully worth it for the door-to-door convenience, especially since we had multiple suitcases. Do note that the boat dock at the airport is a bit of a hike, maybe 15 minutes -- if I had realized we probably would have gotten a luggage cart. I assume the super premium service that the hotel also offered, for ~twice the price, would have someone meet you and help you with the bags. If you are arriving during the day and have minimal luggage (no more than a carry-on), the public transit line from the airport wouldn't be too terrible an option, but be aware that you have to lug your bag up and down the steps of a few canal bridges to get to the hotel (as well as managing it on a potentially crowded boat), and navigating the narrow alleyways of Venice can definitely be a bit overwhelming in general at first, so I wouldn't necessarily recommend it for a first-timer.