Is the GE GDF550PR quiet enough for an open kitchen?

At 52 dB it is mid-range. You will hear it running from an adjacent living area, but normal conversation is not drowned out. If silence matters most, look at models rated in the low 40s.

Does it really sanitize dishes?

Yes. It carries NSF certification for its sanitizing cycle, which uses higher final-rinse temperatures to reduce bacteria. That makes it useful for baby bottles and cutting boards.

How many place settings does it hold?

Fourteen, which is a full-size load for most families. That is on par with other 24-inch dishwashers and more than compact 18-inch models.

What does the steam function do?

Steam loosens dried and baked-on food before the wash, so you can skip pre-rinsing. It is most useful on casseroles and stuck-on starch.

Will it fit a standard cabinet cutout?

It is 23.75 inches wide and 33.375 inches tall, sized for a standard 24-inch built-under opening. Measure your cutout and check the height under the counter before ordering.