This place is legit. Obviously you can tell from the other reviews that the view really sells this spot, but the location/neighborhood can't be beat - you're right where you want to be for a visit to Lisbon. The hosts are great too, very communicative, and provide you with a great list of recommended restaurants.
That aside, the apartment is clean, great amenities, and great AC for the summer. As long as you don't mind a few stairs (which was welcomed after how much we ate throughout Lisbon), this place is very much recommended.