Pete, speaking from experience, it seems that ATM's do clam up sometimes, but I noticed on our last trip, most, if not all worked fine. The worst offenders for ATM's going offline are at the airport in Manila - plus some ATMS have a dislike of UK Mastercard or Maestro, this happened to me a few times, but my Visa card worked fine as a backup. I've never had any credit cards declined at shops or hotels, whether Visa or Mastercard.
I've opened a simple ATM savings account at HSBC Philippines, so I can use that rather than rack up ATM fees on the credit or debit card. I would normally advise against using your credit card to withdraw cash in Philippines (unless its an emergency!), it can turn out very expensive, not only exchange rate fees, but ATM fees, etc. Debit cards are cheaper, Nationwide accounts are free to withdraw cash abroad.