The Temple of Olympian Zeus was the largest of all the known temples in Athens. To give you some perspective, the Parthenon on the Acropolis was about half the size of the Temple of Olympian Zeus. Today, the Parthenon still stands fairly close to its original size, where as the Temple of Olympian Zeus is really just a cluster of pillars. Don't get me wrong, those pillars are impressive, and they give you a great idea of just how colossus this temple once was.
The site itself has two great vantage points, one that is free, and another that costs. Most people's first view of the temple comes when they visit Hadrian's Arch on Vasilissis Amalias. Right behind the fence located near the arch you get a great view of the temple. This view will cost you nothing, and it affords the only view of this temple at night, and it looks great at night. To many this view is enough to say that they've seen it, and not too many people venture around to the other side of the site to enter.
The site itself is a mixed bag. The great thing about going into the site is to get up close to these colossal pillars and really get an idea of how big they are. You can also see the one pillar that fell down in 1852 much better from close up. Other than that this site is fairly barren with a few ancient building foundations scattered around, but not much to see. If you visit the Acropolis, your multi-site ticket does give you access to the Temple of Olympian Zeus, and this makes the site worth visiting.