Thanks to the info in this thread, I employed Synon's service this new year. He was quick to respond by email. Was busy on the evening of our arrival and hence did not provide the 'free pick up' from airport.

Day 1 ($40)
$30 for temple in Angkor Wat and Angkor Thom area + $10 for sunrise at Angkor Wat

Day 2 ($57)
$30 Day charge +$20 for Kbal Spean, Banteay Srei and Banteay Samre +$7 for airport drop

Apart from that, I bought him lunch for both the days ~$6.

He provides free cold water bottles for customers to refresh in his van upon return from temples and keeps engaging conversation.

Overall, I would recommend him for safe and reliable service.

For people on a budget, for temples around Angkor Wat/ Thom area, tuk tuk (~$15-20) would be fine and car is not essential unless in extremely hot weather. For the day trips, definitely employ a car.