We are going to BKK & SR next month. One of the travel books stresses the need for travel health insurance in Cambodia because of the very high cost of being evacuated to BKK in case of injury/illness. We will be in SR 3 days.
We would appreciate your feedback.

You should have insurance even for BKK/Thailand, not only for Siem Reap - for medical and evacuation. Illness and/or accident can happen anywhere regardless your age or current health status. I would suggest you check:

www.insuremytrip.com

They offer various kinds of insurance, from reputable insurers -

1) Comprehensive which includes cancellation/interruption, lost and/or delayed baggage, medical, dental, evacuation

2) Medical only

3) Evacuation only

Check with your health insurer first - you may already be insured for evacuations. By the way, Bangkok has an excellent hospital - Bumrungrad. There was a post on this a few days ago.

Decide what kind of insurance you need. I carry medical evacuation insurance, but do not buy other travel insurance. My health insurance is good worldwide.

You can buy med evac insurance only very cheaply. While medical evacuations are very expensive, they are very rare.

You should always have medical insurance cover anywhere, minor problems are no big deal, you need the insurance for what it is, insirance against some huge cost because of a major problem, it's peace of mind too.