I honestly use Landmarks, Citadels (which are the best Great Person improvement), and sometimes, very few times, Holy Sites...
When I have an Engineer lying about I prefer to save it until I can build another wonder... I always use my Scientists inmediatly... But alas, I'm most of the times going to a Cultural victory...
Thing is... That I sort of agree... I wish Great Person improvements would have an interesting bonus aside from the static yield, like Citadels...
Like Holy Sites generating more pressure to other civs if built on borders, Custom's Houses connecting the resource they are in (so you could replace camps and others), Academies improving a percentage of food, Manufactories increasing the ammount of strategic resources if built on one...
BUT, Landmarks per se already have something interesting, as they can be built by Archaeologists on digging sites, as a defensive way of saving it for later or if you're not interested on the artifact...