I would think it's safe to say that the "comments" field is *always* read by the person at the front desk that checks you in when you arrive. This explains why many people report "it works!" when requesting rooms away from the elevator, opposite the highway, and so on. But just asking for these things yourself at check-in may be equally effective.

The real question is how often these requests are read before your arrival, and how far in advance? Some properties may read them in advance, but only on the day of arrival. This would help in most cases, as they could block a room for you. The high-end properties (such as a Four Seasons or similar) almost certainly read them several days in advance so that they can take care of things that might require preparation (such as "I want room 678", which might require jockeying check-ins several days in advance).