Does anyone have any experience with Bill Me Later? Is it well secured? It's certainly advertised as such and very popular, but consider this:

To use "Bill Me Later," shoppers need only provide their birth date and the last four digits of their Social Security Number -- rather than a full credit card number. (To authorize a purchase, I4 Commerce also needs the purchaser's name and address, which it can glean from transactional data.)
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The last four digits would be much easier to phish than a full SSN. That doesn't sound very encouraging to me...

However...

Does Bill Me Later® protect me from unauthorized charges?
Yes, Bill Me Later® provides “zero fraud liability” protection; the same protection provided by most major credit cards. This means you are not responsible for unauthorized charges.

And Bill Me Later® has been designed with other features to help protect against unauthorized use of your account. Your identity is validated with top-of-mind information such as date of birth and last four digits of your Social Security Number. This means there is no account number that can be lost or stolen.
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Not sure about this one...