Tiwanaku is a site with fascinating elements, but the sum of those elements do not result in a fascinating site. 

The individual elements, such as the Gate of the Sun, the monoliths, and the heads in the semi-subterranean temple (photo) are striking artifacts.  For some visitors, the Gate of the Sun, with a crack in its lintel that adds to its mysterious allure, alone may be worth the price of the admission.  However, other than the few interesting elements still standing (or fallen), the site is monotonous.  

Additionally, the site is heavily reconstructed with non-original materials.  The walls of the Kalasasaya - the platform on which the Gate of the Sun sits - is not original with the exception of the taller stone pillars.  Old drawings and photos of Tiwanaku show the stone pillars standing freely, akin to the ancient stones of Avebury.  It is very likely that there were indeed stone walls between these pillars, but those stones were long ago taken from the site to be used for constructing buildings elsewhere, e.g., the church in the nearby village.

The Akapana "pyramid" is also mostly a reconstruction.  Indeed, Akapana was never quite completed in antiquity and looked more like a natural hill until recently (A Spanish chronicler said of Tiwanaku, "They build their monuments as if their intent was never to finish them.")

There are two museums at Tiwanaku.  One of the museums contain artifacts such as the artificially elongated skulls of the Tiwanku upper-class.  The other museum contains the 7 metre-tall Pachamama Monolith, which was originally set inside the semi-subterranean temple. 

In summary, Tiwanaku is a site that is undoubtedly worthy of its UNESCO World Heritage designation.  For the average traveller, however, it is likely a site that is worthy of a trip if you are already visiting the region, rather than a site to plan your trip around.