My daugher goes to ESF Kindergarten in Tsing Yi and loves it. My impression is they seem to focus more on the socialization of the kids - playing with each other, making own choices, helping themselves instead of relying on the teachers, etc - and less on academic exercises and studies than the local schools probably do. I am often amazed at how mature she has become, especially for a 3yr old. Whether that's good or bad, I don't know.
The biggest drawback in my mind is that they don't have enough Mandarin or Cantonese instruction, but I guess that's not their focus. This may lead us to consider enrolling our youngest in a local (or at least one that is more bilingual) kindergarten when it is time for her to start.
As for choosing am or pm sessions, if you enroll early, you may have more of a choice.