We crossed the border to Poi Pet in April 2012, here are some of our experience and tips for other travellers.
1. As you say the price for visa at the border is 20 dollars. The guides will stop you at a café a couple of miles from the border where you will have lunch. They will try to sell a visa to you and tell you it is much faster this way. This is not true, they go through the same process as everybody else and in the end you will end up waiting for them to give you your visa while everyone else is already done. Tip: Do it yourself at the border.
2. The visa process at the border is really straight forward. Don't worry about it. You will need ONE passport photo and the 20 dollars for the visa. The officials will ask you for an extra 100 Baht when you apply. Note that this is a skam, if you don't want to pay this just smile at the guard and ask for a reciept. Don't seem rushed or angry, just smile, be polite and insist.
3. Be prepared that no matter which way you chose to go (buy visa from the guides or do it yourself) it's going to take a while. There are two steps: 1. Get the visa. 2. Get the stamp in your passport. There will be lines to both of these. Be prepared, bring water, don't be frustrated and laugh at all the signs of corruption you will see (it's really a magnificent display
)