Reaching Siem Reap with a bus can be a good alternative to an airplane. There are many companies that drive you from Bangkok to Siem Reap, but not all of them are legitimate and honest. In Thailand and Cambodia the so called "scam bus" is a well know practice for ripping off foreigners.
Let's look at it in details. The main zone of Bangkok where you can find these "deals" is near Khao San Rd. You can notice that the prices for a single ride to Siem Reap is around 100B (a little more than 2 USD). That should ring the bell, because if locals can't find a ticket for less than 300B (6.50 USD), a foreigner should pay several hundred (foreigners pay always a little more than locals, in Asia and almost everywhere). But they will not only pretend that the bus ticket is so cheap, they will also tell you that they can get a VISA for half of its price, or a discounted entrance ticket for Angkor. All lies.
But if you bought a ticket and you are on the bus right now, you will notice that the fraud just begun. The driver will drive extremely slow. Why? To arrive in Siem Reap as late as possible. Why? They can dump you in a guesthouse of their choice and inn a nice commission from you. Obviously, these guesthouses are often lower in quality than the average.
How to protect yourself from this scam bus? A few rules: if it looks too cheap, it's probably too good to be true. Don't let yourself being scammed: travel with public Thai buses when on the Thai side, and share a taxi with other backpackers when in Cambodia. And remember that, Siem Reap has just finished to renew its airport, and companies like Bangkok Airways or Siem Reap Airways can bring you here in less than 1 hour. The ticket will cost you 60-70 USD (taxes excluded), but you will be sure to be in better shape (and mood) to visit Angkor and enjoy all its magic.