I've been using email since they started accepting it several years ago. The response time varies widely. I've used it for ressies, banking, getting points statements, PS's and so on. Sometimes they're fast, sometimes not. Lately it's taken 2-4 days to get a response, but sometimes I've had a response within a few hours.
For me the bottom line is: if it's a high priority, time-sensitive item, I call; if not I email.