Poipet is the gateway to Cambodia for many overland travellers coming from Thailand. It does not provide a very warm welcome. Gordon Sharpless notes that "Poipet more or less rhymes with toilet" and this caustic observation is, sadly, true. Poipet is a miserable huddle of touts, beggars, thieves and dodgy casinos for daytripping Thais, and spending any more time than absolutely necessary is not recommended.
For reaching Poipet from the Thai side, consult the Aranyaprathet article.
Cambodian visas on arrival are available here with a varying degree of hassle. The border crossing is open from 7am to 8pm. Arrive early in the morning to avoid queues, particularly at mid-day, when the tourist buses arrive. The paperwork is very simple to fill out, and requires no assistance, regardless of what any touts may tell you. Forms are available at the counter to the left of the visa window, although it's likely that a tout, seeking to establish a relationship for later, will bring one to you as soon as you approach the office.
A passport photo is required for the visa; that's the easy part. The hard part is the cost. A sign posted by the Cambodian government over the window of the visa counter states clearly and unambiguously that the visa costs US$20, and that sign is the bane of the visa officer's existence. What he does when you hand over your $20 varies from day to say. He'll usually decline and ask for 1000 baht instead, which works out to almost US$30. If you agree to that, you'll have your visa within five minutes. If you hold him to the price on the sign over his head, though, he'll probably ask for anything from 100-300 baht as an expedite fee, and if you decline that, he'll angrily tell you the wait is "long time" and stare off into space for a while. In that case, you can expect your visa to be done about two hours later (or when his lunch arrives, whichever comes first). That's another reason to arrive at the border early in the morning. In the late afternoon, as it's getting dark, he knows you want to get to your onward destination that same day, and therefore he has the leverage.
Other scams often reported here include having to pay for SARS forms and touts attempting to charge for 'helping' you with your visa application or telling you you must exchange Cambodian riels here (at very poor rates). Don't believe it.
Once you have your visa, brush off the touts and head down the street to get an entry stamp into Cambodia. Compared to the visa, this is a relatively straightforward procedure.
Free buses or shuttle vans should be waiting around the corner from the entry office. These travel to a transportation depot about 1km away, and deliver you into the hands of the Poipet travel monopoly. Tourists are no longer able to negotiate directly with drivers - the drivers know this, and travelers have reported being followed around Poipet by police officers to ensure they aren't able to strike up any deals away from the eye of the travel monopoly, who get a cut from every fare.
Seats on the official bus service from the depot to Siem Reap cost $11 a pop, although departures are not frequent - you may have to wait a few hours, the ride will take around six hours, and you'll probably be delivered to a commission-paying guesthouse for a fresh round of hassles.
Toyota Camry taxis to Siem Reap will take you from Poipet for a fixed US$60. (That's $35 for the driver, $10 for the government, and $15 for the travel monopoly.) The taxis seat up to four people, so don't be shy about introducing yourself to fellow travelers at the visa office and joining up for the ride. The ride along the unpaved highway is bumpy, to say the least. Figure on three hours, but note that travel times may increase considerably late in the wet season (September to December), with some risk of sections being completely impassable.
Taxis going the opposite direction may be cheaper, if you can keep the middle men out of it. Ask your guesthouse. Since far fewer people head to Poipet than away, you have a better bargaining position.
By pickup truck
If you can find them - they're persona non grata near the border these days - pickup trucks connect from Siem Reap and Battambang, although you will more likely than not have to change at Sisophon. Seats inside/outside the truck 5000/3000 riel to Sisophon, plus 10000/5000 onward to either Siem Reap or Battambang.
President Airlines  have declared that they will start flights from Poipet to Siem Reap and Bangkok. However, if you have the money, a much more attractive option is to fly over Poipet. It's a 35 minute plane ride from Bangkok to Siem Reap compared 10 hours in travel by train, bus and taxi, including three hours on one of the most brutal roads you're likely ever to experience, and not counting time you'll waste trying to get out of Poipet with your skin and wallet intact. Even if you're on a tight budget, consider what an entire day lost on these hassles is worth to you - let alone the peace of mind of knowing you haven't lined the pockets of scam artists. Money may be precious, but so is time and peace of mind.
Once through immigration, you will be besieged by touts offering transport. Do your best to ignore them. Unfortunately, for the time being, the only option is to take the free buses to the transportation depot. Do not spend unnecessary time in Poipet if you are not ready to be hassled, scammed and frustrated to the limits of your patience.
See & Do
With gambling being illegal in Thailand, Poipet's location has made it a popular destination for Thais longing to risk their money on roulette and poker. Poipet's gambling industry is growing fast and there are several large, opulent casinos in town, in rather disgusting contrast to the begging children near the border. Other than gambling, whoring and lowering one's opinion of humanity, there are no sights or activities in Poipet.
The best memento of a trip to Poipet is leaving it.
Eat & Drink
There is a convenience store not far from the border in Poipet, although you'll incur some suspicion by heading into town instead of taking the bus to the transportation depot. There are cold drinks and some snacks available near the depot while you're waiting, and some (overpriced) beer and soft drinks next to the window at the visa office.
If at all possible, overnight in Aranyaprathet instead.
Poipet is not safe, particularly at night. Watch out for pickpockets and snatch thieves. If you've managed to arrange a taxi away from the monopoly, don't pay up front, and do not let anybody you don't know into the car. The small upside to the travel monopoly is that, once the exorbitant price for the taxi has been paid, they're reliable, and the driver will take you anywhere you like once you've reached your destination.
You are advised to seek local advice (or check the web) regarding road conditions between Siem Reap and Poipet before attempting the journey. The road condition varies rapidly.