Of all the temples we've been to in Cambodia, this one was the most spectacular. Mostly because of its location on top of a cliff. Because of its remote location, there are not so many tourists so its a quiet place. It's big enough to spend a couple of hours.
There is currently no admission, but you do need to get a ticket. They want to see your passport at the ticket booth. At the ticket booth you can arrange a moto for $5 to get up the mountain. It's a long and steep road up, you don't want to walk this.
There are a lot of military in and around the temple, but we found them to be very friendly and good for a chat. They didn't ask us any money or cigarets. It's very weird to find a shooting hole with sand bags at the cliff end of the temple.
We found it very disturbing that local staff and vendors just through all their garbage over the cliff's edge. Have a look over the edge (on the side), it's virtually a garbage belt. What a shame. I know this is what Cambodians do everywhere, but I can't get used to it.
Although the view from the cliff can be spectacular (or stupendous as it said on a sign at the site), it was not really the case when were up there. My guess is that it should be great in the wet season, but in the dry season there is simply to much dust and smoke (they burn the fields) in the air. We could hardly see the fields at the foot of the mountain let alone anything beyond that. The road is now very good. So you can easily get here in any season.
Note that you can also visit where the ancient staircase starts at the foot of the mountain at the side. It's signposted on the main road just past the ticket booth. It just something for a quick visit, but it's fun to see. If you walk up a few steps you can actually see the crumbling ancient steps go up. There is a new wooden staircase there as well so I suspect that you can actually walk up to the mountain from here.

Prasat Preah Vihear clearly and easily justified itself as being one of the best places I have seen so far.

When I found out that it was possible to visit the temple on a day trip from Siem Reap, I immediately grabbed the opportunity while the site is again peaceful and accessible for tourists to see (note that this place has some serious history of crossfires by Cambodia and Thailand sides earlier). Though it is about 3 - 4 hours away from Siem Reap via rented car, I felt that this trip was totally worth it. This site never failed my towering expectations!

Prasat Preah Vihear - in my opinion - rightly deserves to be on the same ranks as Angkor Wat and Bayon, if not even better. The incomparable beauty of this place stems from its (1) history (older than most in Angkor; dedicated to Siva and, according to some sources, is also one of a few that has a history of critical lingam worshiping), (2) location (situated right beside a cliff to a height of nearly 600 metres on top of the Dangrek mountains, which is within a Phra Vihan National Park of Thailand), (3) relevance (a major pilgrimage site for Khmer kings, as well as a rare key temple off-route the Angkorian Royal Road), (4) architecture (the extensive layout is very unique, the galleries surrounding the central sanctuary served as inspiration for the arrangement of Angkor Wat 300 years later, and the carvings offer a different style from that of those in Angkor - notice the style of their nagas), and (5) the struggles and controversies associated with its inscription in 2008.

I personally like this place as it is not heavily trotted by tourists - thanks to its very remote location. When I went there, there were only about 5 other people visiting the place for the five hours that I was there checking everything that can be explored. Aside from the breathtaking view from the top, I truly enjoyed the experience of being blessed by monks in the sacred central sanctuary, as well as that of checking the interior of a largely ignored tower called 'The Long Haired Lady Prasat' that reminded me somehow of Ta Prohm. Also, I felt the Cambodian/Khmer pride to be greater from this site than in Angkor - this impression is best captured from a photo I got of the iconic first gopura with the three flags of UNESCO, Cambodia, and the World Heritage behind it. Prasat Preah Vihear together with its brother temple on top of Phnom Chisor in the southern province of Takeo, which I also got the chance of visiting back in 2010 with my family, will always have special places in my heart for the value and experience these two sites were able to provide.

I took the service of an efficient driver/guide named San Park (e-mail: sanpark.angkor@googlemail.com) who also gladly took me to several Khmer Rouge-related sites in Anlong Veng. I truly appreciated his punctuality, safe driving, and familiarity of the place and its history; hence, my strong recommendation. There is no entrance fee to the temple. But from the base office, visitors have to pay for the motorbike that will bring them to the top for a fairly reasonable price.