We've stayed there twice, and like it, but Patron is right about the location. It is possible to walk to the Acropolis (we've done it) but it's a looong walk, and would be very very hot and unpleasant in July. It's also quite a long walk to the nearest subway station. And the worst aspect, for those who might like to walk, is that the sidewalks vary from bad to non-existant. Not pedestrian friendly at all.
The shuttle bus's last run is about 7:30 pm, so that's a minor problem for going to dinner downtown or in other areas with interesting restaurants. But taxis are cheap and generally reliable.
The Marriott's own restaurants are pretty good. Somewhat bizarrely to my mind, there's a Polynesian one that's quite highly rated (and expensive) - but it always seems a shame not to have more scope in one's dining.
The staff is pleasant; the Concierge Lounge is nice, and has good breakfast things and hors d'ouevres. Not much else to add.