Preah Khan is one of the most beautiful and best temples to visit in the Ankor Wat complex. The second one being Ta Prohm.
Its reminiscent of all those Tomb Raider & Indiana Jones movies you saw when you were a kid.
Here is ware you see the power of nature and how it can swallow human history. But visit quick. For better or worse Archaeologist are slowly restoring this temple, meaning they have been slowly shopping the trees to pieces that have become one with manny of the buildings in this complex.
As for getting there like much of the rest of the Siam Reap temples you need to rent a tuk tuk driver for a day. Best way is to split it between a few people. But for the ridicules price, mines was $25us a day with tip, you can get to so many other places than the normal over drowned in tourist temples. Bike could be an option if your fit, there is lots to see and lots to bike.
More photos of Preah Khan visit... http://en.wikipedia.org/wiki/Preah_Khan