Personally, Prambanan is one of my favorite temples. I never saw anything more beautiful! It's hard to believe the temple was constructed by humans.
Several years ago, due to a massive earthquake in Yogyakarta, several areas of the temple have been closed off for restoration. However, tourists can walk through the complex, entering some of the temple areas. I was shocked to learn that the massive earthquake in Yogyakarta was the same one that generated the powerful tsunamis throughout South East Asia in 2006. It's effects were devastating to the region.
For the areas of the temple than you can enter, be careful climbing the stairs! They are steep and it's dark inside the temple too. I wore a skirt and it wasn't easy climbing the stairs, while holding onto my skirt & camera. But it was completely worth it to enter the temple. I took a photo with a statue inside. I couldn't believe I was inside a temple constructed centuries ago. It was an amazing feeling.
While tourists can enter some of the temples, unfortunately the Shiva temple was closed off. Many want to enter the Shiva temple because there's a statue inside that you can touch for good luck! Regardless, however, the entire complex is amazing and worth the visit.
While at Prambanan, take your time to enjoy the moment. There's a lot to see and to take photos of while at the temple.
For those of you interested in more information, here's some background information on the temple from Wikipedia:
If you get a chance to visit Yogyakarta, I would recommend visiting Prambanan.