I haven't done this raid, but have heard good things about it. I recall meeting an aussie guy in a bar in Siam Reap one night, who had just finished this trip and raved about it.
Admittedly second hand info, I know, but he thought the set-up & riding was impressive.
The other option is to hire a 250 in Phnom Penh, grab a map and do it all yourself! There are a couple of bike rental places on Monivong bvld (Lucky Lucky and another one next door) that do 250s, mainly old Honda Bajas etc. Previously tourists could not hire bikes in Siam Reap, although this may have changed in the last 2 years.