Coach from SR to Bangkok is easy - two and three quarter hours to Poipet, fifteen minutes through Cambodian immigration but nearly two hours through Thai immigration. Then fairly quick to Bangkok (we went to Koh Chang).
Yeah, it's a breeze from Siem Reap. The Thai immigration has always taken 5-15 minutes for me, but as boris says, sometimes, especially in the high season, it gets very crowded. Once on the Thai side, you can take a bus or train to Bangkok, around five hours. If you leave early, you can be in Bangkok in the afternoon.