However, after visiting the ruins with and without a guide, and with and without Robin's guidebook, I've come to the conclusion that one must choose one's guide carefully. Ours was a friendly, polite, and well-meaning young man whose English was unfortunately not so good. I could make out much of what what he was saying if I listened carefully, but often I just didn't want to spend the attention this would require that I could be spending on the ruins themselves.
At dinner that night we were discussing how he'd been nice but disappointing overall. I described him as "unintelligible, yet strangely meaningful". He was, in that way one can be when every other sentence includes references to the Mahabharata, the Ramayana, the various dieties of the Hindu pantheon, and the Buddha.
I also realized that I really prefer (as obra once said) my life-experiences unmediated, and so when I went out to Banteay Srey with Robin's guidebook in hand, I found a got a great deal more out of the experience.
BTW, monkeygod, thanks for all the site recommendations! I'm pleased to say that I managed to see everything you suggested on a three-day pass, nearly all by bicycle. (Banteay Srei and Banteay Samre were a moto ride away.)
And of course, I had some tasty amok.