Nothing to write home about
We stayed at the San Diego Marriott Marquis over 4th of July and frankly were a bit underwhelmed. We normally stay at another hotel downtown, but it was booked, and we wanted to stay somewhere that would allow us to see the fireworks show over the Bay from our room with our young son, so we decided to give the Marriott a try. 
First, the good:
The location is great -- directly on the bay, connected to the convention center, an easy walk to the restaurants and bars of the Gaslamp district, as well as attractions in Seaport Village, and a trolley stop nearby for longer trips. There's also a water taxi that picks up directly behind the hotel and takes you across the bay to the Marriott Coronado for $6.
The hotel's public areas appeared clean and well decorated, and our King Bayview room with balcony on the 25th floor of the North tower had a great view of the city and bay was nicely furnished and recently renovated. It had a small refrigerator (not mini-bar) and a loveseat/sofa bed, both of which were nice touches but not listed in the description of the room on the Marriott Web site. It had a shower only rather than the shower/tub combo we expected, as well as a flat screen TV. Only two bath towels for the three of us. Otherwise, a standard size hotel room with a small balcony and two chairs.
The pool area was spacious and great for kids, but crowded and a bit hectic over the holiday. Free wifi in public areas, but $12.95 in your room, which always turns me off. The train noise others have complained about really isn't that bad, and we could see the tracks from our room. There are about two freight trains that come through at night, and they cause most of the problems, but the areas is slated to become a quiet zone soon so trains will not have to blow their horns at every crossing.
Now, the not-so-good:
This is a big conference hotel, and definitely felt that way. We didn't experience any of the exceptional customer service others have raved about. We were able to check in a bit early, which was nice, but were not granted late check out. About a week before I arrived, I inquired about booking a suite to give our family a little more room. I was told I had to call the "suite coordinator" as these couldn't be booked online or through the normal reservations line. OK. The woman there was very helpful and told me there were plenty of suites available. I held off on booking as our plans weren't firm yet, but inquired again when I arrived and was told all suites had been completely booked for three weeks. Huh?
There were always long, frustrating waits at the elevators (maybe separate elevators to higher and lower floors would help). You had to sign out pool towels, and then sign them back out when you returned them, which seemed a little unnecessary. The hotel provided a list of activities for kids over the July 4th weekend, but half of them never happened. 
We ate downstairs at the Tequila Bar because there was supposed to be live music, as listed on the sheet we were handed at check-in, but none of the staff knew when or where it would be, and it finally started an hour later than advertised. The majority of the wait staff we encountered appeared to be newly hired summer help, as they didn't seem at all familiar with the menu, forgot to put in our food order, and service was slow. The food and drinks themselves were pretty good, though, and the hostess was extremely helpful.
We would have had a great view of the fireworks, but a "premature ignition" caused all the fireworks for the 20-minute show to go up in about 9 seconds, which was a huge disappointment for us, as that was the sole purpose of our trip... which of course that wasn't Marriott's fault, but put a damper on the trip.
Overall, it's an OK hotel with a central location and a nice pool, but I think there are better choices in downtown San Diego.