In September 2009 I took the Mekong Express Bus from Phnom Penh to Ho Chi Minh City.. I reserved a seat at travel agency and was picked up at the hotel and taken to the bus station by the bus company. A small box with snacks was included and there was one stop for about 30 minutes for lunch. The bus had air conditioning and a toilet. The boarder crossing into Vietnam was painless as I had secured a U.S. visa before departure. Lots of room for baggage and a decent selection of travel times, but I recommend an early morning bus. The bus station in Saigon is centrally located.