I've never asked for a suite upgrade - and I'm still waiting for my first suite upgrade (~200 nights per year in a hotel). I've the same working pattern as described by the most posters above: arrive late in the room (~19:00), leave early in the morning (~06:30, after breakfast). So I also prefer that suites are given to guests who stay longer and/or work in the room. More important for me is a room on the non-smoking floor.
During holiday I often stay longer in the room or come back several times a day to relax. A bigger room may be a plus but superior would be a room with a nice view (and late check-out if necessary). Breakfast is always (leisure or business trip) very important; I rarely book a room without breakfast included. Lounge access during leisure trips would be nice.

I rarely rent a car (I prefer public transport + cabs). Car rental has become quite expensive in recent years. Though the upgrade of the car is free of charge, there are other higher costs as a result of the upgrade e.g. gas (gas in Germany is expensive).