I have stayed at this hotel literally hundreds of times over the years, on my weekly trips to Silicon Valley. It is my top pick for SV stays (with the Hilton #2 and the Hyatt #3). It is old, some parts of it need updating, but it provides a consistent "Marriott experience," which for a business traveler has advantages.
Pro's over my #2 and #3 picks:
Better food. Parcel 104 is good enough for foodies, and the sports bar is good, quick and typically not crowded. Room service is, well, it's room service in a business hotel, but it's usually better than room service at the Hilton, and is more consistent than the other two.
Better Internet. More reliable WiFi, faster, fewer dropped connections. The Hilton's WiFi is very unreliable.
Better service. The few times I have needed non-standard help, e.g., a forgotten item or an emergency fax, the Marriott staff has shined vs. the other two. The on-site UPS store has come in handy a few times as well. Unfortunately, the Marriott recently downgraded the on-site store to a closet, so if you forget toothpaste you have an easier time buying at the Hyatt vs. the Marriott or Hilton (actually, at the Hyatt you wouldn't need to buy it, since it comes free in the room).
More upgrades. I am the top level in both the Marriott and Hilton frequent traveler programs; I almost always get a free upgrade at the Marriott and almost never do at the Hilton. If this sort of thing matters to you, the Marriott is better.
Con's vs. the other two:
Parking: it's the only one of the three that charges ($9 a day), and if it is raining you have no choice but to get wet walking to the hotel. The Hilton has much better parking (free, closer, especially if you park in the back), the Hyatt does not (if you don't get one of the few spots out front, you are stuck parking in the convention lot and then you have a long walk to get into the hotel).
Room: the Hyatt has the best rooms of the three, but the Marriott rooms are fine, other than the bathrooms, which are small and old, especially compared to the Hilton. The Hyatt also has small bathrooms, but there were upgraded substantially when the property changed, a few years ago, from a Westin to a Hyatt.
Bottom line: all three are good choices for a business traveler. With a family I would probably pick the Hilton (bigger rooms). For sports, the Hyatt now definitely wins, with the new Giants stadium being built in the hotel's back yard.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.