Air tickets nowadays pretty cheap. I remember when I went there about 7 years ago, it cost me SGD700+ just to fly to Phnom Penh. Then took a 6 hours "under the HOT sun" "above the deck" boat got roasted like BBQ chicken to reach Siem Reap.
An alternative route, which might be pretty interesting for people who have time, take a flight to Bangkok, then a bus to the border of Cambodia, can't remember the town, but it's a casino town, then a 3 hours bus to SR. You might need to stop over at the casino town for a night, but hotel there is cheap. And you got a chance to see the border of Cambodia and Thailand, where mornings hundreds or thousands of cambodians cross the border to Thailand to work or for business.
There are alot of things to see in Cambodia. Flying there for a 5 day trip to really enjoy Siem Reap is not enough. Take a 3 or 5 day pass to the ruins and take your time to walk though all the temples, understanding the history why and how the temples were built will be a very worthwhile history lesson. If your aim is just to shoot the sculptures and sunrise, sunsets, then 2 day pass is enough. But there is much more to a place which has about 500 years of history, which was once one of the most powerful kingdoms at it's time.
There is a town called Batabong (I think that's the spelling...), pretty nice and quiet town, where you can take a river taxi from there to Siem Reap and see the boat people living along the rivers.
Get hold of a Lonely Planet and plan your route from there. In all these years, I've never used the local agent to book my trip. Calling the guest inns listed in lonely planet and using google for search is a good tool to get hold of your accomodation done.
Best time to visit Cambodia is year end and avoid monsoon. But if you are a seasoned backpacker, anytime of the year is good. You get to enjoy different aspect of the country at different times.
