Siam Reap to Bangkok Border Crossing

Ask at your guest house .. they will know a taxi driver that goes to the border and will be able to come and pick you up or will tell you where you have to go.

There is a place in Siem Reap that the shared taxi leave from - last I knew a year or so ago it is was Psar Thmey.

An update on my experience crossing at Poipet. I still couldn't get a good answer about where the shared taxis left from. I found a tuk tuk at 6:30am and told him what I wanted to do. He stopped along the river not far from the road to Phnom Penh. There I got the front seat for $15, which normally seats two people. As I was discussing what I was going to do, 4 Cambodians came and took the back seat, so we left within 15 minutes. Much shorter than I had imagined.

The ride took about 4 1/2 hours and the road was definitely not good in places, lots of mud and then very bumpy roads. At one point, we picked up one more person, that ended up sitting in the middle of the front seat, but with his legs on the driver side - very careful not to take up any of my space!

We dropped the 4 Cambodians in Sisophan and then headed to the border. Once through, I expected to be hounded by drivers, but not one came up to me. I wandered around and ended up getting a mini-van right at the border for 300 Baht that was supposed to take me to Lumphini park which is a lot closer to Sukhumvit than the Northern station, plus, I didn't have to pay for a driver to get the bus station at the border. I ended up getting off a little earlier because a traveler I had met, said it would be closer.

All in all, it took me just over 10 hours from Siam Reap to Bangkok and was not a bad experience at all.

Good to hear that.
The taxis leave from the junction of NR6 and the bus station road. There used to be some at the Koh Ker Restaurant on NR6 in front of the lights near the river, but don't know if they're still there.
Price you paid was ok, but 4 1/2 hours means the road has deteriorated. I once did it in 2 1/2 hours.