Preah Vihearr is about a 3hr drive from Siem Reap and well worth the effort to get there. The drive is very pleasant as you pass through some gorgeous countryside on the way. We went with San Park who is a very safe driver, reliable, knowledgeable, has a good sense of humour and speaks good English. He can be contacted on: firstname.lastname@example.org. I would highly recommend him.
Take your passport with you as there is no charge for foreigners (at the moment anyhow) to visit the temple. The return cost to go up to the temple is either $5 each on the back of the moto or $25 per group per ute. We were so glad we took the ute option as it allowed us to take wonderful photos as we wound up (and later down) the hill. I think on the back of a bike I would have been hanging on for dear life!
You are also encouraged (by the driver of the ute) to buy a carton of cigarettes to hand out to the army guards on the way up the hill. This is a very strange thing to do but we decided to go with the flow and the soldiers were very friendly as they collected them.
We spent 2 hours at the temple. I am not a huge historic temple person but this one is well worth the visit. Make sure you go to the very last level as just past this there is a brilliant view out into the valley and Thailand. The temple is spread over several levels and is long and narrow (nearly 800mtrs long). There are also quite a few lovely shady spots to sit down and have a cool drink (sold by several vendors there) if you need.